1. Lacking support for old GPUs in a maintained driver. nvidia drivers approach for old gpu is "use an old driver" for cards as recent as the 6xx.
2. People like to run fully open source systems
3. Adding support for things like kms and wayland are really up to the whims of nVidia as to when it happens with the closed driver, while an open driver is more likely to do so (or at least can be forked to do so). In the past features like config-less X were also much later coming to nvidia drivers which made it harder for people to get started on Linux.
Your point #2 is especially important: with all the commotion surrounding the various "management engines" (in effect a completely independent system with privileged access to the carefully secured main system) it is surely obvious that a powerful GPU running unverified code is a foolish thing.
Point #3 is also really good. Who wants to wait around at the mercy of a corporation which has an active interest in NOT being transparent about "its" IP (as speculated elsewhere most of the GPUs probably infringe on some or other patent)?
Anyone buying an nvidia card for Linux should know exactly what they are doing: funding an anti-Free Software company, deeply entrenched in the ethos of secrecy, patents and bullshit; saddling themselves with a white elephant which will suck hours of their precious time.
2. People like to run fully open source systems
3. Adding support for things like kms and wayland are really up to the whims of nVidia as to when it happens with the closed driver, while an open driver is more likely to do so (or at least can be forked to do so). In the past features like config-less X were also much later coming to nvidia drivers which made it harder for people to get started on Linux.