Probably has old incompatible hardware. But there are imo other legitimate reasons, specifically if you have < 2GB of RAM (or don't need more).
1. If you have limited storage (for example a 60GB SSD - commonly used as boot drives and in laptops), x64 Windows takes up SIGNIFICANTLY more space, leaving less room for apps and games. I have both an x64 and x86 box - Windows on one is 23.5 GB, and on the other just 13.6 GB. That is the equivalent of one whole AAA game. This should also, in theory at least, impact loading performance (e.g. boot times), but my hardware is far too different for me to verify that. Anyone tried it?
2. If you have slow internet (e.g. modem), and/or tiny data cap (common in many countries outside of the US) the drivers (e.g. graphics drivers, Windows updates etc) are SIGNIFICANTLY larger (generally around 40%) for x64 Windows, wasting precious bandwidth. Case in point is the Nvidia Geforce driver: for x86 121 MB, and x64 169 MB (or 40% larger).