The March event will be about AW 2, iPad Air 3, AND the 4 inch phone. Why would Apple also lump something so important and big as the redesigned Macs when there are three other products being announced?
Apple did it before at the Spring Forward 2015 event. Redesigned MacBooks right along side the hot new Apple Watch. The products you list are already established....an entire keynote about a thinner AW, a thinner iPad Air and a smaller iPhone can easily be rounded out by announcement of a thinner rMBP (or more powerful, or whatever). WWDC can be reserved for for Mac Pro and iMac updates.
The 13" has already exceeded the previous longest; now 304 days and counting (will be a full year if it comes out in March). If Apple delays the 15" in the same way, that makes it a June release.
Even better. We're basically in uncharted waters with the 13" but looking at it from two different perspectives. The fact remains, in the past 10 years of MBP -> rMBP development (not including the non retina 13" MBP which is supplanted by the 13" rMPB), there has never been a full year between updates. While some are are passing off June dates as if its the obvious answer but fail to realize that it would be an unprecedented length of time for the entire product line. For the 15", since Apple releases are within a year, Theres no reason they can't be in March or April. May is unlikely and June, like I said, would be unprecedented..