The amount of configuration required to rival what's available out of the box in Web clients (especially indexing, searching, filtering, blocking, etc.) is a bit too much for someone who isn't already interested in it. I tried GNUS in Emacs and a few TUI apps; I do like having all my emails accessible locally, but for day-to-day use, web clients are more convenient. I haven't tried Thunderbird or Outlook (if it still has a local version), though, so maybe I'd have all the conveniences I want there - but since I already have them in Fastmail, I just don't have an incentive to switch.