Please, there are probably 700 fixed gears in Boston alone. (a slight exaggeration, perhaps, but not by much). Most people don't go to CMWC.
On top of that, urban riding is not just about fixed gears. In fact, if you look around, even in the most fixed gear crazy town, they're probably the minority of bikes on the street.
There are enough urban cyclists to sell at least two books specifically on the subject in recent years.
Personally, though, I hope nobody ever comes up with such a beast. If by "urban cycling" we mean straight up transportational cycling, there's not much to discuss. Gear for getting around and being able to land at a place where normal people hangout, parking tips, not getting hit by cars tips, getting around the city on off-road paths, and probably a few other topics. It seems like it would be a bit thin unless it was a very irregular magazine.
Now if you make it a little more like a skater mag, it could involve trials type stuff and all that.