Microsoft stopped supporting XP in April, 2014. That's almost five years ago. AH was one of the last mohicans to support it.
There's several reasons why applications stop supporting old operating systems. One that should be obvious to even the less computer savvy is internet security: If you get a nasty virus that steals your identity because you're using an unsupported system to run an up-to-date program, whom could you sue? Right, not the company who officially has informed that their system is no longer safe to use!
Another reason is compatibility. Microsoft no longer provides tools to create apps for platforms from XP to 10. Of course one might be able to use the old tools for XP and the new ones for the rest, but that would mean an entirely different version for a marginal clientele. You want to play against
all AH players, not only against those very few using XP!
The current statistics tell that in the USA 94% of Windows desktops use a supported version and 1% use XP. Last night there was about 100 players when I logged in. Compared to those stats that would mean that there would have been one player on a dedicated XP server. Now
that would be low numbers!