I was an Aztec C user. That was the second compiler I ever purchased (after Abacus C for the C64). Damn... I'm still buying compilers today thanks to Microsoft.
(I work on open source stuff and I've only ever used GCC and Clang - I'm not asking this as a rhetorical way to say you're stupid, I'm genuinely ignorant of the advantages).
2 - The IDEs are good. Not great, buggy sometimes, but they help you with the more hairy things in Windows.
3 - Even if it's something like C# that would be easy to do with the Free IDEs, it's helpful
So, to sum it up, developer convenience.