These things get planned out 3-4 years in advance. The Larrabee decision was made in the last couple of quarters.
I wasn't involved in the decision, just the results, so I can only speculate, but I suspect it was done to allow flexibility in stepping the "chipset" die and "CPU" die separately. The design groups for both have been separated in the past, and I think for this first foray into the merger it was considered most expedient to allow for the most flexibility.