Actually, all of the quad-core Core i7 processors have a TDP of 45 watts. So it's probably not a power consumption concern. More likely, it's a concern about Turbo Boost. As seen in the attached Wikipedia screenshot, quad-core mobile Skylake processors with Iris Pro graphics can only boost one core's operating frequency at a time while models with HD Graphics can boost all four cores' operating frequency simultaneously.
I think it would be price as the main reason. Or shipping date. Or the fact the Iris Pro CPU is 100MHz slower.