For the same reason everyone writes other financial systems for Windows. It's a popular platform with rich frameworks and a large developer base. The current v

For the same reason everyone writes other financial systems for Windows. It's a popular platform with rich frameworks and a large developer base. The current versions (>= Windows 7) of Windows are pretty secure. Even the last patched up version of XP isn't awful. That said, my experience with ATMs has been that nobody does a good job of hardening ATMs.