The issue with the Macbook Pro update and specifically the 15" one is that they really don't have all the CPUs for the 15" one to upgrade with. The top end of the CPU line the "HQ" quad-core processor.... I have seen no information on when there will be a Broadwell processor (if there ever will be) and Skylake is still a bit away. For a "full" refresh (and not an incremental) they really need the new CPUs for the entire line of laptops.
I believe Broadwell was rumoured for summer 2015. But when it will actually launch is anyone's guess.
Maybe Apple know something we don't, though. After all, they could have given the 15" MacBook Pro the new trackpad if processor updates were a long way off (more than a year), but they held out.