Engineers, and especially the hardware kind, are a limited resource. Not that Apple couldn't spend even more than they currently are on growth, but they're reaching some limits, and until later this year office space is/will be one of them.

This is combined with the fact that the Mac Pro sells in the "low single digit" percentage range of Mac sales means that it's hard to justify an outsized expense. When factoring in R&D costs, my guess is the Mac Pro has much lower margins than other Macs.