Welcome to the new world where Apple cuts costs as much as possible even thought it is swimming in cash.
The same rational is why they refused to bother updating the Mac Pro for about 4 years... and the new one isn't exactly coming out until the fall.
Or, alternatively and with slightly less tin foil involved:
1) Apple has used the same part because its primary function is to get some distance between the wireless antennas so would look more-or-less the same anyway.
2) The new Mac Pro isn't coming out until the fall because Intel hasn't made the Ivy Bridge Xeon CPU's yet! Not that hard really... As for why they didn't launch this with an earlier generation of CPU I'd suspect the cost of flash was the big problem, there's just no room to fit a hard drive and large amounts of fast PCIe-based flash has been both horrifically expensive and usually built around full-size PCIe cards. Oh, and they really need the increased bandwidth of Thunderbolt 2 to drive all the potential external devices they've highlighted and that isn't available until later this year either.