The kinetics matter here. The B2 flies much higher than the C-130 which would aid the GBU-57 MOP (almost certainly used here) in it's ability to penetrate to maximum depth. 80% of the 15 ton weight of that bomb is just heavy metal to give it maximum energy as it borrows into the ground.

Also, each B2 can carry 2 MOPs making it a better platform than a C-130, and that isn't even taking the stealth of the platform into account