Because you're getting LOADS more motherboard for the same price, the 1156s clock much easier than the X58 stuff, dual channel is naturally a lot cheaper and, if hyper-threading isn't important to you, a i5 750 is way cheaper than a 920. Yes the X58 is amazing. But the P55 is much better value.
This Maximus III is a mental amount of motherboard. The serious contender for it in X58 terms is the Quad Classified. And if you're trying to say they're the same money....