At least give us 30GB free, 15GB is just obnoxiously low for how much value G data mines from inboxes.
(0): https://en.m.wikipedia.org/wiki/GMail_Drive
Edit to Add: Apparently the storage wasn't unlimited? At least from a cursory look at Wikipedia. Huh. Memory is a funny thing.
The main reason for success was that Google was still generally seen as a not evil company, while other mail providers were garbage/evil.
https://www.cnet.com/tech/services-and-software/gmail-accoun...
Google later changed it to “unlimited” which increased daily by an MB or so. At some point settled at 15 GB.
https://gmail.googleblog.com/2013/05/bringing-it-all-togethe...
Not sure if it was unlimited actually before this point.