The number of MMO's at the time were limited. But as a kid, the idea of playing a game for free, with the cash shop being 100% optional, is a big selling point.
Most games I played at the time had a perk you were desperately starting to desire, as you continued to play, and when things got harder. Eventually, you'd reach a point where cash users outweighed the free ones, and you either grind for money or drop your parents wallet.
That philosophy may be dead now. But at the time, that was a major attraction point for many people I knew who played it.
The gameplay was a step up from Runescape and Neopets at the very least. Which I might add, are both still running, despite being free to play.