Vulcan was crushed and sucked into the black hole. It didn't "blow up". Shielded starships fared better. Not difficult to comprehend.
As for time travel, that can be applied to EVERY SINGLE episode and movie in Trek. Even Nemesis - why didn't they go back and undo that? What about Voyager - why wasn't the timeline repaired after the events of "Timeless" and "Endgame"?