The other comments hit the big ones (didn't have to download, had a free tier, didn't have significant irreversible character choices, had meaningful solo or quasi-social activities, etc), but I think a major contributing factor was it appearing on miniclip.

Others have said it but free to play option and it ran in browser. I had no money and knew WoW was more popular but my low end pc couldn’t run WoW very well and I couldn’t afford the $15 a month subscription.

I think I did end up paying for a RuneScape membership but even then it was only $5 a month.