Because PC is a better platform in every aspects. Sure, it requires more efforts, but in the end, it's also a better experience.
First of all, you have a unified ecosystem across various hardwares AND form factors, thanks to Steam.
It's pretty simple: You want a desktop PC ? You can do it. You want a console formfactor ? You can do it. You want a laptop ? It's also working. Heck, you can even get it on a handheld/tablet form factor now.
If you're not looking for performances or graphics, you can get a cheap machine and enjoy a vast library. You're okay with console level of graphics ? You can get a similar priced hardware and enjoy the same kind of experience. You prefer a higher framerate and resolution ? You can have it too.
Online exprience is also a better one. No paywall, enjoying cross game chat and a great community system.
Prices are cheaper, hardware isn't region locked.
You can enjoy a vast choice of controllers: Xbox, PS, Nintendo... even wheels, arcade sticks, and flight sticks.
As for the library, there's no generations, so you can enjoy backward compatibility with nearly every generations. And I'm not even accounting emulation with that.
I grown up with consoles, and always hated PC. I couldn't simply play on that. But since I build my own 5 years ago, it's just my main platform now. I also think Steam is a major point of making PC gaming an enjoyment. Last generation, I still played on console because of Japanese games... but now, even that is moving onto Steam.
I think that PC and Steam will be important for the market in the future, because we won't have to depend on manufacturers choices for hardware or the way to play.