I think the problem is with the choice of DLC.
There are over 1000 pieces of DLC you can buy.
So say there are 4 million rock band owners on the 360(just as a number example, I honestly don't know.)
And each of these 4 million rock band owners has 10 DLC tracks.
That's 40 million downloads right there, and that's just 10 dlc tracks each.(which we know there are people with a LOT more than just 10.
So lets say you get a group of four heavy DLC buyers.
Say each person has 400 DLC tracks.
The chances that all 4 have even 50 DLC tracks in common are still not very good.
So people are buying DLC, it's just that the way the system searches for players doesn't take into account that you have a certain percentage of DLC in common when it does its searches.
And in case anyone was wondering, if all 4 million of those 360 owners downloaded 1000 tracks each(and lets face it they aren't,...not even close), that would be 1 billion DLC downloads.