There is a reason, they don't want people who don't know their OS is unsupported sending them bug reports of shit not working right and expecting it to be fixed...
This will likely include a simple "this OS is no longer supported, you may not play [game] on this operating system" message when trying to install (or run if already installed), which makes is VERY clear that is is not supported. This will result in no pointless bug report submissions, support forum threads, etc that need to be looked at by blizzard staff and then discarded.
It's a matter of efficiency, and this is efficient.