See also https://sqlservercode.blogspot.com/2016/11/what-was-first-co...
https://www.ami64.com/product-page/kcs-power-cartridge-c64
See also https://sqlservercode.blogspot.com/2016/11/what-was-first-co...
It's been a while, I should rewatch it
The Great Bridge: The Epic Story of the Building of the Brooklyn Bridge by David McCullough
https://www.amazon.com/Great-Bridge-Story-Building-Brooklyn/...
The main reason for success was that Google was still generally seen as a not evil company, while other mail providers were garbage/evil.
UI-wise, yes, but GMail was also the first mainstream "AJAX" [1] app. Before it the expectation was that clicking on almost anything on the page would cause a full page refresh. GMail was in part revolutionary because of how snappy it felt for a webmail client.