The only reason I can think of is so they can phase out Windows XP and sell more copies of Windows 7 and 8. At the cost of developers all over the world.
XP has been out of mainstream support since April 14 2009. XP SP2 mainstream support ended on July 13, 2010. XP SP3 mainstream support is still in effect. That's the phasing out indicators, not what platform .NET runs on (in my opinion(
However mainstream support doesn't mean "We're going to target our newest software against an operating system that's 10+ years old."
As for having to test on 4.5, well, 4.5 fixes bugs. You'd also have to test against .NET service packs for exactly the same reason I'm afraid.