More because web apps are awful. Even Gmail is awful and god knows how much money goes into developing that.
GMail is bloody awful. I can hardly stand using it. Been using Thunderbird for a while now and have not many complaints.
The whole "IMAP" purist thing is unnecessary; there are plenty who prefer not using the GMail interface and have no idea what IMAP is.
Telemetry, activity tracking, etc.
NeoMutt doesn't send everything I do to a persistent log accessible to data broker parasites and NSA functionaries.
Not being turned into cattle watched over by an enormous Panopticon is a pretty major Killer App feature.
I like that it has critical features like a sane approach to contact lists and “sweep”. I also like that I can send a message with priority - a feature mysteriously still missing from gmail.