Unfortunately, it's also mostly what many fans know of his stuff(I was annoyed once to hear someone call him "that guy before Garth Ennis," when Delano had handpicked Garth as his replacement). Vertigo, with its highly inconsistent reissues of that run(only now finally done right), did not help, as for a long time all you could get in proper order was Ennis, with one token Delano volume of his first issues.
And again, Delano's stuff is very much for grownups, both in subject matter and sensibility, so that doesn't help broaden his audience. It's hard to explain quickly, and doesn't fit into trends like, say, Morrison. Or grab people via gross-outs, like Ennis. Or have a balance of cute and scary, like Gaiman(who also fits well into the fantasy genre as a whole). Or isn't considered the Holy of Holies like Alan Moore, whom he should be considered almost the equal of. And he doesn't do superheroes, like, well, all the others. If there were more Ramsey Campbell, Burroughs or JG Ballard fans in comics he might do better, and at the height of the Vertigo era there were that sort and he did better. He's the odd man out, and I think that's why he's ignored.