Let me explain why. Last night while watching for the second time, if I heard Jet Fire correctly he says "only a Prime can kill the Fallen". I think some are taking this out of context and assuming that only Primes can kill Primes. He is only referring to the Fallen. Then after Prime dies the Fallen says "finally the last Prime is dead". This tells us that Megatron is not a Prime. If Megatron was a Prime why would he not take out the fallen? Megatron would not bow down to anyone if he knows he could take them out. The Decepticons are the ons that find Sam and get all the info they need out of him.