My guess is that there wasn't enough energy left on the planet to unseal Buu. SSJ2 Gohan only filled half the required energy. SSJ2 is what? 100x Base? 200x Base? Either way, I'd say it's at least twice as strong as FPSS or USS, so Trunks would probably only fill a quarter. Combine that with the fact that the population has been reduced to the thousands, and I doubt there is much Babidi could do without traveling the universe for more energy, which could possibly expose him to the Kai.