Because until recently, on Linux AMD's driver story hasn't been good and performance has lagged. The driver situation is improving but it will take time to win people back. It also doesn't help that they don't (yet) have a Ryzen-like story on GPUs.
Does the new driver architecture support controlling fan speed yet? I've been on both team green and team red, and not being a purist I have just used their proprietary drivers. It wasn't until AMD shifted to their new driver architecture (with most people suggesting to use the "high quality" open source one that required baking firmware into my kernel (I run Gentoo)) and I had so many issues with both the new open source one and proprietary one that I accelerated my time-to-update-card timeline and went back to NVidia, emerged nvidia-drivers, and have never had a problem since. Nvidia has been rock-solid for me on Linux (even when I first had an old card in 2009-2012 or so) using their proprietary drivers. There's also the continuing fact that while you can get 2 AMDs to match the performance (and cost) of one Nvidia, I'd rather just have the one card every time.
Don't know as I'm right there with you using nVidia myself right now. While I'm going to be picking up a new Ryzen-based system soon, AMD's GPU offerings aren't where they need to be for me to shift away from nVidia yet.
