One problem is the lack of a large HL2 level reviewing website. I remember Radium was around from day one when HL1 was released and was a great resource to find quality levels. Level authors knew to submit their new levels to Radium, and HL1 gamers knew where to find new levels. Granted, the reviews were just one person's viewpoint and he may not have been right all the time, but it succeeded in getting levels out there and shone the spotlight on the better levels.

HL2 on the other hand - are there actually any HL2DM level reviewing websites at all? The only one I can think of is Ascendany, and even that is just a sentence or two on each levels, hardly a full fledged review. http://www-personal.umich.edu/~fischmt/ascendancy/

This isn't helping with HL2DM's popularity. Just about the only way to find quality levels is to trawl through forum posts and wade through dozens of newbie or "just made for fun" efforts.

Whilst this isn't really answering Myrk's original question, I suppose it has some bearing since when creating levels for HL2 they aren't likely to be played as much even though they take longer to create (in my experience anyway).