The reason people keep using XP are probably:
Vista was terrible so that broke the chain for many.
Most computers are still running from the XP days and are fine. If it ain't broke don't fix it.
Windows 7 - well, XP seems to be working fine.
Windows 8 - Oh dear

In the banking sector in the UK all the old mainframes are biting the dust, nobody know Cobol or how the old programs works (All the programmers are long gone) so they can't transfer the data...

For me I still have to support XP, so I can only ship .NET 4. This is a pain.
P.S. I believe there is a Cobol.NET
