This is pretty simple really - unless Apple wants to launch a new Mac Pro and have it be out of date almost immediately, they need to wait until Intel has released Ivy Bridge Xeons, which won’t be until next month at the earliest (and given the delays with Haswell, July seems unlikely). Also coming later this year on Intel’s roadmap is the introduction of Thunderbolt 2.
Both of these things would seem like an excellent foundation for a new line of professional Macs.
Given the very short list of hardware model numbers that leaked ahead of today’s WWDC keynote, my guess is that Apple is going to hold a pro focused event in 2-4 months and refresh MacBook Pros, Mac Pros and hopefully the surrounding halo like Thunderbolt displays (which are crying out for the new iMac style case, the newer non-glossy screen, USB3.0 and soon, Thunderbolt 2) and the pro software Apple sells.
Having a pro-only event would also help calm the worries that Apple has stopped caring about high-value-low-volume professional users.