I wonder how much of that is simply due to the ability to use the Gmail API vs IMAP.

I’ve been knocking around the idea of building an email client, but IMAP looks like nightmare fuel. JMAP would be perfect but adoption looks like it’s going nowhere.

[edit] Seeing a lot of people here diss on Gmail. I don’t blame you, I want out too. If you want more options, lean on your email provider to start supporting JMAP. https://jmap.io