Because it’s not that they can resurrect anyone - Ben was specifically able to bring Rey back by giving her his life force, but he had to sacrifice himself to do so. The fact that he’s able to do it at all makes sense because of their rare force bond which goes all the way back to Force Awakens when he was probing her mind and she was able to prove his at the same time. That connection was built on in Last Jedi and developed even further in TROS.

Honestly, the connection between Ben and Rey along with Ben’s story arc and redemption is the one thing that the sequel trilogy has gotten right through the three movies (at least until the unnecessary and grown- inducing kiss.) And you was definitely thinking he was unredeemable after killing Solo. But when you look at the three movies that’s the one thing that actually does look like it was planned out and works.

I’ve got plenty of other problems with this movie though.