Ugh

Maybe people can't run Black Mesa. Or they don't want to.

Also, isn't Black Mesa incomplete?

Maybe they want to play the mods for it? Or they want to play the original also?

The mod isn't finished, it's missing the last area. Black Mesa is more graphically intense and may not run on older/weaker systems. The mod is quite a bit larger (+required Source SDK), and some people have to pay for data or have bandwidth caps, so downloading anything large isn't worth it.(it's also still bizarrely not downloadable through steam still despite being Greenlit) This game is a piece of gaming history and one may want to study it, flaws and all. Lastly there are mods for original Half-Life that people may want to play.

Your question was perfectly reasonable, I'm just answering it.