Why do you think they're the only one, don't you think maybe they enjoyed a very special set of circumstances that will, more than likely, never happen again?
I think the only special set of circumstances is that they were able to go public and remain independent. At this point most companies get acquired as an exit, but thats not just Open Source companies, thats almost all companies.
The fact that they were the first to support an infant OS that will end up powering the majority of the server out there is a stronger reason imo, but you're right, it's definitely a reason. Another one is the internet bubble of course.
IIRC they weren't the first (Caldera...?), but they were among the few that managed to survive the dot-bust. One reason they survived was that they were (then) charging less aggressively than others, at a time when money dried up everywhere. When the bloodbath was over, only them and SuSE were left standing; and SuSE was eventually swallowed and mismanaged by Novell.