Tsunade was a known senju, had proficiency with water release and earth, had groundbreaking chakra control and even had some Uzumaki in her blood she literally would have been the perfect character for wood style, so why Moegi?!
Tobirama ticked all of the boxes as well, but having multiple wood style Hokages would be lame. I prefer that they all have their own styles personally. Also, why is this in the versus section?
I haven't read Boruto manga yet, but it says in Wikipedia that she used Mokuton because she combined Doton and Suiton, and she has no blood relation to Hashirama. I'm guessing it's possible for Sharingan or Rinnegan users to apply this method themselves then.