The main thing gmail offered back then was a frickin GB of email storage. (Which spawned a host of other things, like a FUSE filesystem backed by Gmail.) No other free provider was even close! Hotmail offered email measured in 10s of MB and I'm sure Yahoo was similar.

(I got my first gmail account in 2004.)

Yeah, the UI wasn't bad, but there was less email back then and I didn't (still don't) really mind being served HTML repeatedly instead of some "AJAX" application, which is what I remember the technique being called. (It used to be the latency to the server was poor and JS hid that; now, on 2-3ms fiber, it seems that the JS actually introduces a lot more latency than it hides.)