We are trying to be consistent. We've heard from customers that when products like vCenter and SRM have different compatibility matrices, it creates a lot of confusion. It prevents customers from "standardizing" on certain OS, a feat that reduces operational hurdles (and saves money). What we wanted to be able to say - with 5.1 U2 (for ESXi, vCenter and SRM) Windows 2012 is supported. And by "supported" we mean "tested".
Please note that SRM will refuse to install on an unsupported OS - again not because of technical reasons, but because the installer knows it's not been tested in this configuration and if there are indeed problems, customers could not easily get help.
Hope this answers your question
Stefan