Cause they weren't coming back. Marcus had the Enterprise sabotaged. They fire one torp, maybe a two, and their asses are covered up with Klingon cruisers and blown straight to hell. It was a one way mission that was meant to get Marcus his war and clean up his mess all in one shot.I think that it should have been explained better why Khan went to Kronos and why Marcus kept Khans people in the torpedoes. I don't think the answer is obvious or even logical.
I still liked the movie but to me this is its biggest flaw. Not the Caitians, not the call to PrimeSpock, not Carol's underwear.
Also why they get and accept 72 torpedoes to kill one guy running around on a planet.
They wouldn't have fired 72 cruise missiles at Osama bin Laden, just 1.