I recall Monte Cook saying something to the effect of that they were originally planned to be in the original 3e monster manual, but were held back for page space reasons and not wanting to glut the book with extraplanar critters. At some point down the road, later design team members either intentionally deviated from that, or were never aware of it, and the modrons largely dropped off the map.
I also seem to recall a statement by Andy Collins from a few years back, after that point, where he said something to the effect of "everyone thinks they're stupid"*, as a way of explaining why the modrons had little more than a paragraph in the 3e Manual of the Planes. *maybe based on market research but who knows.
A few years later still there was a question about them raised during the designer chat for the 3.5 Planar Handbook (the one that was rather interestingly censored before WotC posted the transcript), where there was a joke made about the modrons all being dead.
And then Dragon magazine featured the awesome modron article by Ken Marable with the modron as the cover art. I figure by that point since WotC already knew they were going to be doing 4e, and probably already had plans to scrap the 1-3.x planar cosmology, they frankly didn't care about modrons showing back up in print in the magazine. That's pure speculation on my part however for that motivation.
And given the overlap between the 4e design team and the team that did the PlHB etc, I very seriously doubt that we'll ever see a 4e modron from WotC.
