Hardware and software support mostly. OEMs and vendors often write software and drivers for Windows only, or have better support in Windows.

But there's no reason I know of to keep using XP instead of 7 or 8 for these applications.

Disclaimer: I work for a company that engineers software for self-service patient check-in kiosks.

Because their software only runs on Windows. If it ain't broke, don't fix it.

Driver support for various touchscreens? But that's just a guess. I would imagine the license cost is not a huge factor in the price of an ATM.