For me it is the fact that setting up a GUI client takes 10 minutes and learning to use it even less. If it was the same with mutt, I would switch in a heartbeat.
I tried gnus and mutt in the past, but gave up on them after a few days. I was spending too much time tweaking the config, trying to remember shortcuts or googling how to get it to work the way I wanted to. IIRC even displaying Greek with an ok font was a problem on gnus. That was 10+ years ago, could be that the documentation is better now and a good-enough setup is easier to achieve.