The Senju Kekkai Genkai
I'm pretty sure the senju kekkai genkai isn't actually mokuton, that's just a by product of their actual kekkai genkai which was proably a genetic predisposition to nature chakra.
I believe the mokuton is a specific alignment of factors that correlate to create the perfect circumstances for it to present. Namely, a water and earth nature affinity, VERY good chakra control (which is clearly a trait of the main family senju line, if hashirama, tobirama, and tsunade are any indication), AND a predisposition, or at least a learned ability, to passively absorb nature chakra (which even Naruto couldn't do, he absorbed it actively).
That would explain why the mokoton is so rare despite kekkai genkai usually being dominant genetic traits in large blood-related family groups (Uchiha, Hyuuga, Hoshigaki, etc) because even if all the senju family could passively absorb nature chakra, few of them hit the rest of the necessary factors.
Take for example Tobirama, hypothetically he hit 3 of the 4 required factors to create mokuton (water affinity, chakra control, nature chakra) but because he was missing that 1 single factor (earth affinity) his kekkai genkai presented as an incredibly powerful water affinity and skill over healing and seals*, but no mokuton.
As well as that, despite being much smaller than the Uchiha, the Senju as a whole were able to compete with them on the battlefield. If we assume the disparity caused by the Uchiha's size and kekkai genkai was being bridged by a boost from nature chakra to each individual Senju, the Senju's ability to keep up is much more feasible.
It is also generally agreed within the fandom (never officially confirmed), that when the mokuton becomes too strong, the user gets turned into a tree. Now, that sounds VERY similar to nature chakra imbalance which Naruto was threatened with when he was was learning Toad Sage Mode, adjusted to suit a Tree Sage of course.
It would also explain why so many Shinobi (Orochimaru, Madara/Zetsu) throughout the Narutoverse struggled to recreate mokuton despite having direct access to Hashirama's cells. Unfortunately, I'm not overly familiar with cellular biology, but at least with kidney transplants (with modern science) the rejection rate is about 15% (correct me if I'm wrong, I googled it), not the ridiculous 99% we see when Orochimaru attempted to recreate it under Danzo's orders. Even accounting for the likely uptick in deaths due to adjusting genetic code being considerably riskier then a simple kidney transplant, it is not unreasonable to assume that most of the transfers were probably successful. Therefore, it is possible that the 99% fatality rate was due to the sudden and overwhelming influx of nature chakra into small bodies and minds that did not know how to handle or balance the sudden intrusion. In fact, we even see this when Tenzou's neighbour eventually succumbed to the experiment, and roots and branches grew out of her. Much like the cost that is described for those that wield mokuton, and also an imbalance of nature chakra.
Finally, that would explain why Sakura doesn't have mokuton when, if the show's description of the kekkai genkai is too be believed, she definitely should. Kishimoto's explanation for mokuton is that it is a perfect balance of earth and water nature affinities which, when combined, can cause plant life and vegetation to be manipulated and grow at extraordinary rates*°. However, if this explanation is to be believed, then there is no reason why Sakura should not have mokuton. It is generally agreed (again, never confirmed) that Sakura has earth and water affinities, and her chakra control is considered one of the greatest in the history of the Shinobi nations (canonically the greatest during her time). Therefore, if we're going by kishimoto's explanation, logically, Sakura should be able to use her chakra control to balance her water and earth affinities. However, if we rather consider it with the theory that the Senju kekkai genkai is nature chakra, then Sakura suddenly needs 4 prerequisites, not just 3. And Sakura has never shown any proclivity to nature chakra, so she does not hit the last requirement.
So. Yeah. The Senju had a genetic green thumb that gave them superpowers, not just trees. I think I had more points somewhere rattling around but I've forgotten them lol. I was also going to do a TL;LD but ehhhhhhh too hard. Do with this information as you like, thank you for coming to my TED talk.
*it is generally believed that one must have a certain mental predisposition towards healing and seals to be able to fully master them. Hence why the Uzumaki were so feared, because it was quite literally impossible for any other Shinobi (even other seal masters) to master some of their work because the logical progression (seals work somewhat like code, except WHERE you put each line of code mattered almost as much as WHAT it said) was incomprehensible
*weirdly enough, hashirama's mokuton generally uses pre-existing plants and seeds to create his constructs whereas Tenzou most often creates them from himself. Perhaps less control/balance over the nature chakra? Rather then using it in the traditional sense, he has weaponised the nature chakra's own side effect? Would also explain why his mokuton is weaker and why he's never shown to have Tree Sage Mode. Could also simply be because neither had formal training and simple made the mokuton entirely their own in how they used it lol
°according to kishimoto's explanation, mokuton is more akin to a release, like D's storm release (water and wind) or Mei's lava release (earth and fire), not a true kekkai genkai which CANNOT be recreated without the necessary genetic code, much like most dojutsus', the hoshigaki's sharklike appearances and skills (although that is often argued to be more as a result of their summons), and the hozuki's naturally transformative cellular structure (which is BONKERS by the way)













