Because IMAP sucks on bad network links. It involves a huge number of round trips to synchronize the state, and re-establishing the shared state when the connection is interrupted takes forever.
A lot of online commenters refuse to believe this but the standard Gmail interface is highly optimized to cope with bad network connections, hide latency, and recover from interruptions. If you have the code assets and initial state cached in your browser, it behaves very well under bad network conditions.
yea it's fair that you can just use IMAP and sync before your trip then send after.
but I was on a flight, didn't have Gmail or Superhuman cached and could not get either to even load. I do suspect that if it were already loaded, Gmail probably would have functioned decently well.
still Gmail and Superhuman just seem...bloated. kinda cool to just have a simple, open source interface for the Gmail REST API.