Shoot me, but I'm kinda ambiguous.
I always thought that the reason why they're not as popular as DT/other more mainstream prog bands is that they manage to put out one album within the usual length of the cycle everyone is actively exploring prog in - it takes you approx five years to rejoice at ~100 cool prog albums, mostly from upcoming bands, and one Symphony X album instead of the usual two some other bands would have. They don't make you think of them regularly, with no live releases/extensive tours to where a lot of their fanbase lives, either,