1) It was first.
2) Good balance of generic fantasy gameplay and unique world design. On one hand, its generic enough to cover a lot of different types of gameplay, and it is easily modifiable. Shadowrun, as an example of contrast, can't be used for much besides the world of Shadowrun. On the other, it has worlds and character classes and archetypes that are entirely unique and unbased on anything mythological or historical. So, a wide consumer base is available.
3) Better design, especially in more recent editions. Good emphasis on simple and usable over complex, accurate, and unusable. Further, design complexity scales well depending on how much you want in your game. New players can enjoy the core rules and have a great time. Advanced players can add things like, oh, the new Tome of Battle, which requires a lot more rules knowledge (swift actions? immediate actions? free actions?) but which all vary off the basic core rules themes.
4) The SRD is free. I learned how to play out of it, and now I'm frequently the DM for our games. By reducing the entry cost to playing their games, WOTC lured me in sufficiently to convince me that their game was fun. Now I occasionally buy one of their books. Taking Vampire: the Masquerade as a contrast, the entry costs are very, very high.
2) Good balance of generic fantasy gameplay and unique world design. On one hand, its generic enough to cover a lot of different types of gameplay, and it is easily modifiable. Shadowrun, as an example of contrast, can't be used for much besides the world of Shadowrun. On the other, it has worlds and character classes and archetypes that are entirely unique and unbased on anything mythological or historical. So, a wide consumer base is available.
3) Better design, especially in more recent editions. Good emphasis on simple and usable over complex, accurate, and unusable. Further, design complexity scales well depending on how much you want in your game. New players can enjoy the core rules and have a great time. Advanced players can add things like, oh, the new Tome of Battle, which requires a lot more rules knowledge (swift actions? immediate actions? free actions?) but which all vary off the basic core rules themes.
4) The SRD is free. I learned how to play out of it, and now I'm frequently the DM for our games. By reducing the entry cost to playing their games, WOTC lured me in sufficiently to convince me that their game was fun. Now I occasionally buy one of their books. Taking Vampire: the Masquerade as a contrast, the entry costs are very, very high.