I think it's mainly due to it being such a direct route into town, coupled with the fact that it's turned into the first Superficial Highway. I for one used to use the A23 but swapped to use CS7 when it launched as it only added about a mile to my journey but I found it slightly more pleasant then heading via Brixton!
Yes there are numpties but I think that's just par for the course TBH, much like drivers the majority seem to be of average skill with a small number being at the upper and lower ends of the scale!
As for numbers I had a few comments on my dangerous LGV video that I posted earlier in the week commenting on the sheer number of riders at the lights, I'd suggest it was easily 20+ :-)
Lets not also forget though that the design of it also does encourage some potentially dangerous conflicts, not to excuse the idiot RLJers and genuinely bad riders, but when you have sections where it carries on straight along the main road with turning traffic coming into conflict with cyclists (looking at the area by Kennington Park here, it happens in both directions about 1/4 mile apart), that wonderful piece by Stockwell where you need to get to the middle of 3 lanes to follow the cycle lane and the infamous left hook magnet that is Rookery Road :-)