The Homely Mutt is still a pretty good resource for getting back up and running on mutt. I've had a versioned muttrc sitting around for a long while. Judging from the stuff I found in there when I dusted it all off over the weekend, and then looking into my scores file to see what I was nudging up or down in visibility over the years, I got serious about mutt and developed a pretty complex configuration sometime well before Ben was born.
I know exactly why I quit using it, too: it was a good thing when mutt made it possible to talk to IMAP and SMTP directly and introduced caching, but it also meant that search became a problem: The search tools needed a Maildir or mbox archive sitting on the disk, and all mutt provided was its own cache. I struggled on for a little while happy that I was liberated of the need to use fetchmail to get stuff down to the disk, but hating that I needed to go to the Gmail web interface to search for older messages.
The thing that made me wake mutt back up this time was just plain disgust with mail software generally, and how thwarted I felt when I tried to integrate my text/org-mode based workflows with Gmail. So I pulled down my muttrc, bumped back into the search problem, and decided I was feeling fidgety enough this time to go back to The Homely Mutt and do the parts that involve offlineimap and notmuch.
It took an evening to get it all set up, but I already had a complex muttrc and some of that was backtracking or reconciling the guide's advice with what I was more familiar with. If you're curious, the maildir branch of my muttrc repo has plenty of stuff including examples for scoring, some decent macros, and a method for flipping back and forth between a "work" and "personal" identity with macros.
If you're looking for a why, I guess I'd say that when you have mutt running and set up correctly, it's just kind of a joy. You can customize it to suit your whims, it's super fast, and you don't lose much compared to a modern GUI or web client. Very little, in fact. If you choose to run it on a local machine, you can hook it into system tools that allow you to preview attachments, etc. pretty seamlessly. And when it comes to actually writing a mail message, you can use your favorite editor.
When I sit down to mutt and take a brief inventory of my feelings as I use it, I'm reminded of how I felt about WordPerfect 5's pristine blue screen: It just sat there waiting for you to fill it, it was nimble and clean in a way little else was, and it felt like the last stand of great software from the waning days of MS DOS. Certainly my 8088 IBM XT running WP5 felt much more unencumbered than the 286s running Win 3 and Word I was saddled with at work.
The price of that feeling is a lot of fiddling. Literal hours of learning and experimenting. Also awareness that this is a slowly dying part of the computing world. The documentation is thin, outdated, and often sort of cargo-culty. You're always aware that your upstream services, like Gmail, are sort of hostile to the toolchain you're depending on. You depend on the good graces of people putting together the Homebrew packages to avoid the agony of trying to build this stuff for yourself on something other than Linux.
And after a while, something makes it seem uncomfortable and itchy for whatever reason: I end up needing to do a thing more quickly than the time it would take to learn how to do it in these old tools, maybe, and then I sort of lose the habit and fall back to the easier tools.
Everyone needs a hobby.