Last year I defected from AMD to Nvidia based on availability and bought a 1070ti, after having used Crossfire for years.
As soon as prices are once again "reasonable" (a relative term), I'll be adding a second 1070ti for SLI.
While many reasons have been floated, one that hasn't been mentioned is multiple monitors.
I used 5 with AMD, but it seems that I am limited to 4 with Nvidia, so I actually use 3, although I will be stepping up to 4k versions to compensate later this year.
Crossfire or SLI both allow excellent framerates when using the higher resolutions that multiple monitors provide, and I don't see that need going away any time soon, based on projected card development.
Multiple monitors is a niche, but many of us still use them, so Crossfire/SLI still fills that need to be able to play at the same framerates with multiple monitors, as a single card on a single monitor.