Not a novel observation from me but: untraining entails going into diapers 24/7, and the mechanics of being 24/7 — without any specific reference to untraining — themselves encourage untraining and make it more difficult to get out of diapers, for reasons that are both mechanical and organic. In no particular order:
Diapers are physically harder to take off than underwear and you're far less likely to be able to put them back on once they're off.
Switching from diapers to underwear is difficult to pull off without a shower, because even if you haven't been using your diapers, proper diaper hygiene is not really underwear-friendly, and if you have been using your diapers, even if you're very hygienic you're still going to smell weird.
If you're in diapers and comfortable with using them, you have a financial incentive to use them because not doing so is wasting a diaper, and diapers cost money.
If you're in diapers, depending on how much privacy you have you may have an incentive to use them because wetting them is quieter and, at least initially, more deniable than taking them off.
If you're in diapers, you are much less often incentivised not to void than you would be if you were in underwear. This might mean that you can void when others literally can't — for instance if you're in an area with no toilets and nowhere secluded enough to pee. It also just means that it's constantly very easy for you to void when it's difficult for others — you can pee in class, others have to get up and leave.
This tends to have an impact eventually because while someone might not actually want to untrain, it also becomes increasingly obvious that they don't have an actual reason to wait and therefore it becomes easier not to. People tend to eventually take the path of least resistance.
This may be a personal observation, but I honestly think you're incentivised not to take wet diapers off because then they cool down from body temperature and Cold Pee Feels Extremely Weird.
The more layers you're in, the harder it is to get them off. This is true with everything, including diapers.
I've mentioned this before and had the occasional rebuttal along the lines of "tapes aren't that hard to rip off!" or "onesies aren't that hard to undo!" and for one thing, I have impaired general fine motor skills! I'm speaking from experience here! But more generally it's not only a matter of "can the thing physically be removed," but a matter of "can the thing be removed without pain?" and "can the thing be removed without damaging or destroying it?"
If you're in a diaper you want to put back on, the viability of that project is going to be particularly dependent on how you undo it, even with hook and loop diapers: the diaper may technically be refastenable but most medical-grade diapers these days are like one-ply so if you rip too hard you'll strip the hooks off the wing.
You can rip onesies open but I've found that if you apply too much force, even with onesies from well-regarded suppliers, instead of ripping the press stud off the other press stud and hence the flap off the flap, you rip the flap off the press stud (which remains closed) so you have to spend a couple extra seconds prying it open with your fingertips lest you ruin a onesie.
You can rip waterproof pants down but even very good ones are generally tighter around the legs than underwear and have leg bands made out of materials with a higher coefficient of friction, meaning they will be physically harder to simply pull down and will hurt like billy-o if you try to do so. As with onesies, depending on the plastic pants, you also risk damaging or destroying them if you pull them with too much gusto, by way of the main shell of the pant simply separating from the waist or leg bands if you pull with too much force.
There's also individual manual dexterity issues. I have never been au fait with cloth diaper pins or bib-and-brace overalls and while I was regularly using them I had a lot of help changing and dressing. If you're in that kind of situation, that's another wrinkle to consider.
If your wardrobe is sized for your diapers then trying to go without the diapers can be an extremely harrowing experience. Basically anything with a waistband often ends up feeling like it's about to fall off, regardless of how tightly you fasten it (if that's an option).
If you're in diapers overnight, and you have issues with waking up to pee overnight — which seem extraordinarily common among AB/DLs — then you have an incentive to use your diapers rather than get out of bed. However, this also means teaching yourself behaviours which can eventually enable bedwetting.
Control is fundamentally a matter of intra-abdominal pressure (IAP). The muscles of control supply a counterpressure, which prevents voiding even in the presence of a certain IAP, or a certain pressure usually determined in part by IAP (like pressure inside the rectal ampulla). Pretty much everyone's IAP can be elevated to the point that they lose control regardless of their continence level, but most continent people don't experience circumstances that would do this on a daily basis.
IAP is determined, in part, by mechanical compression of the abdomen. Diapers and waterproof pants have higher waistbands than the average underwear and press slightly harder, elevating IAP in precisely the region of the abdomen where this feeds into control. If you have normal continence this might just feel like being squished slightly. If your continence is poor enough that it can be compromised by laughing then the effects here can be more significant.
Being in diapers increases the likelihood that other people will eventually become aware of them, which in turn increases the pressure to remain in them. This does not mean other people will keep you in diapers, nor is it something to be sought out. It is simply a fact of life. Conversely, there are virtually no situations in which someone except another AB/DL or maybe your partner is going to actively correct you if they catch you not wearing diapers. This does not mean that the expectation will not exist.
Accidents of any kind — meaning voiding while intending not to, regardless of circumstances — agitate in favour of being in diapers. Generally, nobody, regardless of their circumstances or inclinations, wants to be in danger of accidents at potentially any time in their life. Accidents outside of diapers also increase the probability that a third party will make it their business (although that probability, both the beginning value and the increase, will vary wildly depending on the third party).
Accidents while in diapers present less of an immediate external risk of shame, but are detrimental to control for other reasons. Namely, people often exercise control while in diapers to avoid diapering tasks that require a lot of effort. Failures of control force the person to carry out those tasks and become accustomed to them, and, usually, to find ways to do them faster and with less effort, thus reducing the person's incentive to maintain control. For instance, someone might avoid messing their diapers because messy diapers take a long time to change. Messing accidents, however, force them to figure out how to change faster, reducing their incentive not to mess their diapers in future.
There's also the problem of diminishing returns, to which I will devote its own post.