I think it's an exaggeration to say "you couldn't throw a stone without coming upon CP".
It was on every single Hidden Wiki at the time.. it _was_ everywhere, and commonly linked to from sites like 4chan.
other drug marketplaces pop up
Sure, but nothing like the Silk Road. In a winner-take-all market like the online marketplace market, you would expect a top dog to emerge.
When did you do your research? The difference I noticed began late 2012.