This is actually a pretty interesting observation as GMail, when it first came out, was just as clunky as all the other webmail clients. At the time, everyone was used to Yahoo!, MSN, etc. and Google was the odd one out with their webmail client.

This changed when they were the first folks out there to get a dynamic interface in the browser (some of you may fondly or not so fondly remember the days of DHTML, XMLHTTPRequest, and the like). Fast forward 10 or 15 years and now GMail is the standard by which everything else is measured.

I'm sure there are some things that are objectively better, but a surprising amount of preference comes from familiarity.