So, Kylie got her hands on a bunch of Herba Mystica and ended up rather huge and powerful, right? Is there any in-universe reason that prevents your various villain characters from getting their hands on even more than Kylie was able to? Or is it more handwaved away as 'eh, not the focus, not my problem' kind of thing?
Basically Herba Mystica until after the events of the Indigo Disk was only found and able to grow in Paldea in certain specific enviroments (usually caves or dark humid areas), plus where it is found is mostly in tera raid dens (which are recent) or where the Titans live. So why the villains from the other regions wouldn't know about it is probably that. Some villains also might not use it because of their own egos (but some do end up using it but thats a problem for later)
There is one villain who gotten more than Kylie did and its where Kylie got her hands on Herba mystica to then grow her own supply. She stole it from the Open Sky Titan, Beafito Estrataleza, a bombirdier who basically hoards the entire supply in the south west of paldea.
There is one event that happens that also prevents them from getting herba mystica and going on a rampage but thats spoilers
So the TLDR is that the in universe reason for why the Apex Alliance doesnt get their hands on Herba Mystica is because
1) its too rare during the events of the story they would be in to be known
2) The titans of Paldea horde it for themselves
Heres a pic so you can know who Beafito is
However, if you want to know whats stopping the alternate versions, i.e. the Twisted and the Antithesis (assuming you know who those are) from getting the Herba Mystica... well... nothing lol