Just because XP is "supported" by the Windows team (even if in a security-fix only state), it doesn't follow that the developer division should necessarily continue to produce newer tools for it, it is fundamentally a dead platform, the tools needed already exist and nothing is going to change that.
* We don't have to support Windows XP because it's end-of-lifed
* Supporting Windows XP is expensive (since we have to test it on more platforms than if we didn't)
* Supporting Windows XP can reduce the performance of our main product (since conditional execution to do one thing on XP and another on new OSes is slower and testing on Windows XP editions is time that could be spent making improvements on Windows 7 editions)
* Supporting newer versions of Windows means we can use newer features (such as slim-read-write locks or NTFS transactions) without having to have crusty code for implementing that functionality on older OSes which don't have them built in.
* There are tools already out there to build executables for XP - so we're not even constraining our customers by stopping Windows XP support in Visual Studio
* If this helps to nudge our customers, or customers of customers to upgrade to a newer version of Windows, we get more money.
So remind me again why Microsoft would want to ship a product that supports Windows XP?