I think that happens to everybody.

Regarding Dream Theater in particular: Pull Me Under was their only "hit." If you listen to any radio station that plays more than just the popular metal at all, that's the one Dream Theater song they'll play. They've been around since the late '80s and are a good band all-around with a very loyal following (otherwise they wouldn't have lasted so long). I've just read that the drummer (a founding memeber) has recently left the band, so there'll be nothing new from them anytime soon.

They are always called "progressive rock/metal." Although, I honestly don't know what the hell that means. The best I can figure (when bands are called that) is that they play, on average, longer songs, with longer periods of no singing; they extensively use keyboards; the lyrics are "philosophical" or serious, rather than about having a good time; and they don't play the same melody or rythym throughout a single song. Why any of that makes a band "progressive" is beyond me.

Ultimately, it's not surprising you haven't heard of Dream Theater, because they don't make the kind of music that is going to get a lot of radio play.