I played 6 exclusively for about 20 years. That was back when I played live all the time. My reasoning was basically muscle memory. My concern was that swapping between different numbers of strings would inevitably cause mistakes, due to momentary confusion and brain farts. I am sure there are others that can do it no problem, but for me, I was always concerned that I would accidentally step on my crank live. Heck, I didn't even like having different numbers of frets for the same reason.
Now, since I don't play in a live setting too often, and mostly just record, i swap between all manner of strings: 4 string bass, 8 string guitar, 6 string guitar, 5 string bass. But truth be told, I do occasionally get baffled and forget where I am or what instrument I'm on. But when doing a live project, I will try to stick to one bass, so there are less possibilities of any of that happening.