Well, because it provides offline, DRM‐free installers for your games, often including Linux installers.
GoG ostensibly cared about Linux in the beginning, but by now it's very clear that they don't.
It's their choice of course, and among all the alternatives to Steam I guess they're the best choice.
But they obviously don't want me as a customer, so I largely ignore them and don't recommend them to anyone.
In almost every single aspect, Steam is just the superior product.