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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.
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It was free to play when that wasn’t a popular financial model for games at the time. The memberships were also much cheaper than WoW and accepted different payment methods that would be more available to minors at the time.
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Also no need to install or commit to a guild or clan or whatever. You could just log in, screw around for an hour or ten, and log off. Then log in again from somewhere else. It was miraculous for a kid like me.
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Could even run on the computers at my middle school, nobody thought to block it
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It's free and you could play it in the school library, for one. Also it came out a few years before WoW.
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As a kid, I didn't have a computer that could run WoW, nor would I have been able to convince my parents to pay for the subscription. I suspect most of the popularity was from kids in a similar situation, probably playing it on library computers when they should've been studying.

MapleStory was also quite popular back then for similar reasons.

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Besides what everyone else said, it was a pre-WoW style sandbox MMO with a lot of freedom at the time. Those have their own appeal.
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They were two very different games and target audiences, with the incidental commonality that both are MMOs.
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No they were actually not different audiences.

WoW got everyone

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WoW needed a purchase, an expensive subscription, and a good PC. Runescape ran on a potato, was F2P, the P2P was 5$ a month, and you could get started in 2 minutes from Miniclip.
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in 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.

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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.

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