alt Hacker Newsin addition to what others said...
2004 runescape, now "old school runescape", which is actually successor 2001 Runescape "Classic", were both playable in a browser.
and my (parent's) PC couldn't even install, let alone run WoW. despite me trying with all my might.
so I'm sure I wasn't alone.
it was absolutely magical to me.
Yep. You could play it almost anywhere. School, friends house, parents work laptop. It required little hardware and, as you said, ran in a browser.
Meanwhile, WoW had to be installed, took a fairly large amount of space (I remember 14gb on my 28gb HDD), and Ironforge ran like garbage on my PC.