Adokat: I never really played 4x at all until recently, although I think you're wrong that they aren't successful. Sins of a Solar Empire had good critical acclaim and financial success. Civ 5 is one of the top-5 most played games on Steam, and maybe the top singleplayer game, so it's clearly found a big audience.
I couldn't get in to Civ4, but I really love Civ 5. I think that made me more interested in 4x games. I bought the MoO pack from here, and I'll get to them soon.
Phosphenes: I don't mean that they're never really successful, I just mean that they've never really been very popular. A lot of people don't even know what a 4X game is. Civilization is really the most popular, and most people call that a turn based strategy rather than using the term 4X.
Well, I guess we have different definitions of 'popular.' Strategy games as a whole are never as popular as an MMO, FPS, or RPG, but they do quite well for themselves. If you want to include the total war games as 4x, then think most people would be familiar with the concept, as that series is also quite popular.