I think it was a convergence of luck, the solving of a specific problem that was relevant at the time (sms messages), well timed celebrity adoption (Shaq was an

I think it was a convergence of luck, the solving of a specific problem that was relevant at the time (sms messages), well timed celebrity adoption (Shaq was an early twitter user) and the idiosyncratic constraint of 140 characters which distinguished the platform from alternatives and contributed to a unique feel. It's large part luck, but the particular features play a role, too.