Because it was probably cheaper that way. The manufacturers are passing the security buck to the banks themselves, and folks who work in banking IT/infrastructure don't generally know enough to understand what these machines do. As long as the cash counts in the cassettes matches up with the receipt at the end of the week no one is worried.
May be it is a selection bias, but as unlucky customer I had seen hundreds failed ATMs from various banks on streets and almost all were with XP Home, exceptions were XP Professional, Linux, Windows 98 SE, OS/2 and DOS.
I am from Russia, so corruption may be explanation.
