The whole discussion about defining the greatness of a band is kind of a dead end. Even by success standards. I could argue that DT simply came out at the wrong time. I could argue that had they been around in the 1960s or 70s, they would've been the most successful band to have ever existed. Even if their music would have been different because they would have had different influences, the type of skill and musicianship, in combination with the type of creativity that they possess would probably have yielded something truly great that would've left bands like Zepplin in the dust. Instead, the time when they did come out was pretty bad for their genre of music, what with the proliferation of Grunge, and popular Metal evolving into the genres of Groove Metal and Nu Metal. One could easily blame those factors for the fact that DT never got the kind of popularity and recognition I think they rightfully deserved.
By the same basis, I could argue that had the musicians from Led Zepplin (or Beatles) been a couple generations late, coming out in early 90s, for example, their style of music would have probably never been really noticed. They came out in the hey day of Rock'n'Roll and Hard Rock, not to mention, before the time when things like tape trading or music downloading existed, so naturally they would've yielded higher album sales, because at the time, if people wanted to listen to their music, there would have been little choice.
And I'm not even going to bother talking about how the different decades, different sociological factors dictate how certain types of music might be received in those eras, because that would be a long and drawn out thesis.
Of course, it's all hypothetical speculation, and we could only guess, so it's fun at best to imagine "what if" but otherwise pointless. Bottom line, when it comes to a band's success and cultural impact, there are so many factors at play that it's simply unfair to say, "They were successful just because they were a truly great band. And DT isn't as great of a band, as proven by their success and impact."