I think you're right regarding x86 performance being a huge factor. Also my own experience, admining a number of Solaris systems in those days, I just found it more frustrating to use. The first thing I would do was put GNU userland on there to make a Solaris system more usable.

Later on when I had more say in what systems things ran on, I'd reach for Linux because it was easier. It was good enough. I could easily run it at home too, and it ran on cheap hardware.

I sometimes wonder how much that usability factor played a role...

Though AWS seems to be doing fine as a business, so maybe this theory about usability isn't so great.