From what I've seen others say who have been in contact with the "big names" in modding apparently Suckthesda decided to make changes to the engine that made modding way more difficult than it was in the past and changed how some file structure works so combine that difficulty with an already super lackluster game and they basically gave up on it.

Bethesda shot themselves in the foot big time by paying lip service to modding, but actively making it worse.

My money is on money. The same reason Paradox make Collosal Order dumb down cities skylines 2 modding capabilities. They want to sell you content, not let modders create it for free. Once they get their dlcs out then they'll open modding up to whatever degree it's possible with the changes they've made.

How transformative it is is debatable, but the total war warhammer series got a really strong modding scene that's only gotten better with time.

Bethesda keeps their engine for a reason, there is a lot of people that will be intimately familiar with the modding tools once they arrive.

Also like… do we know it will even be easily moddable

They will be releasing a toolset same as their other games. They just haven't said when other than sometime in 2024.