One word, expandability. The top-end iMac is pretty much maxed out, whereas the low-end MacPro has plenty of room for expansion and performance upgrades.
If I was in the market for a new high-end development platform, I'd be all over the low-end MacPro and then upgrade it myself. It's definitely cheaper to get memory and HD's from Newegg than Apple. Although I do have to say the 27" iMac is pretty sweet but I doubt I'd have enough desk space for it.