I think a lot of publishers do look at the sales of D&D vs. d20 Modern (over a million core books sold vs. perhaps 50,000) and think they'd rather go for the wider audience. What really matters though is the number of active purchasers. While there are less d20 Modern fans, they tend to be more active purchasers because their game of choice has less support. Right now, a Modern fan can easily keep up with everyone support product that comes along, whereas that stopped being the case for fantasy d20 books in 2001. As Chuck said, less fans but also less competition. d20 Modern has been a lot better for Green Ronin than I expected honestly. I was ready to do Ultramodern Firearms and call it a day (funny, because I had the same attitude about Death in Freeport in 2000!), but we are happy to keep supporting the game.
Something that may have held d20 Modern back is the lack of an ass-kicking campaign setting that resonates with a lot of people. While there are umpteen things you can do with d20 Modern just using history and the real world, that approach puts the onus on the GM to do the research.
We are hoping that our upcoming offerings, the Red Star Campaign Setting and Sidewinder: Recoiled, will give GMs what they need to make the most of d20 Modern.