Because the moment anything was done that didn't quite fit into to canon, a significant portion of the Trekker universe would try to blow up the innerwebs... just like they've done with Discovery. Yes they would.
Unless CBS All Access and the production company in charge of this hypothetical show released a statement explaining the show wasn't in the Prime Timeline but actually set in a new, matured timeline, they would need to find an in-universe explanation for the differences or else feel the fanboy wrath.
Yes, I know other franchises have successfully done alternate versions of the same characters without having to explain why they're different, but as fireproof78 put it, this is Star Trek.