I find Den-O VERY annoying sometimes really. It's not good writing, it's "fun" writing rather than "good" writing. It's comedy. That's why people like it. It may not be a piece of art like Blade Faiz or Agito, but it brings a few smiles. But I have to say that after Episodes 20+ it gets fuckin boring with the same old people cracking up lame jokes.
Kuuga and Ryuki were really good stories, but the reason it wasn't popular among kids was the fact that the way it was written must be a little complicated for kids to understand. Den-O was a simple story and it kept it that way without being so complicated. It was simple, sometimes fun, and enjoyable at some point, but it's no expert material.
Blade is one heck of an awesome drama itself. Hibiki actually taught me a thing or two about growing up. Kabuto was kinda sensitive and dark at some points. Kiva had it's drama which wasn't so bad. Den-O isn't like them. I'd actually put Kiva with Den-O, cause Kiva's story wasn't so complicated either, and the humor they tried to squeeze in sometimes wasn't funny. I mean, when I'm watching it, and something people laughs at, my ace is just you know, blank, I don't find things most people do funny. Though Kiva wasn't really "fun" writing all the time like Den-O.
I geuss it's just easier for the kids to enjoy a comedy show than a serious awesomeness, full of epic drama and battle/fights, and also easier to understand. Try letting a kid watch Kuuga then Den-O. He'd probably be confused with "Go-Babu-Go-Bingzatzobu" or the language, and only enjoy Den-O.