Probably because he feels he can be a lot more useful in this universe, especially with his efforts as an Ambassador to try to bring peace between the remaining Vulcan's and the Romulan's at this time (albeit on the very very down low if he's logical about it.) Seeing as he knows of what's about to happen to Romulus in a century and a half, and the knowledge he possesses to possibly stop it then he can show some good intentions this way in this universe. Once his work is done, and he hasn't been hit with Bendii Syndrome (I know I probably spelled that wrong,) then he may consider a return to *some universe.*
Do We know for sure that the Romulan Home world will be destroyed in the future of NuTrek
Wouldn't it be more logical to think that Spock-Prime
would do something now, to prevent yet a Third Alternate Time line
Will Nemo even be the same personality in this reality???
Isn't speculation to a migraine level fun????