For various reasons I think.
In terms of the number of vendors, it all begins with the explosion of the RC market with Mephedrone. Even before all the new RC vendors appeared online, Mephedrone was appearing around clubs & it took off from there. As it gained popularity more & more people jumped on the RC bangwagon. I think probably because of the fact it was legal & the profit margin. I mean the mark up off even small bulk quantities was like 400%. I knew of two people who started dealing locally just for that reason never mind online. The media hype surrounding it all only compounded matters, probably bringing it to attention of even more people whether they were using or selling it.
The fact that Mephedrone gained so much popularity amongst people who had never taken RCs before & even amongst people who had never taken drugs before meant there were a lot of people taking drugs that had no idea what they were using or the risks. It's also led to them branching out & trying other potentially more dangerous drugs. (GBL springs to mind) As such, there were a lot more people being hospitalized which the media latched onto as it would sell their papers. There was a lot of poor, uninformed reporting but it brought the RC market to the attention of the wider public.
After Mephedrone got banned, they just kept trying to put out new RC's in the same manner trying to replicate the success of Mephedrone. However, with the UK Government now saavy, anything even remotely successful that came to their attention would get banned including their analgoues, which has made it harder and harder for the online vendors. As such, we are getting many weird, wonderful & quite possible dangerous RCs appearing.
I really wouldn't associate crime with the RC market. It really was just every man & his dog who was selling RCs. That doesn't mean there weren't people also involved in criminal enterprises who branched out but I don't think it starts with them. I've no doubt they have carried on with the likes of Mephedrone though. I would be shocked if the RC vendors who are savvy enough to be finding new RCs would be involved in anything illegal. I mean what's the point; I'm sure they make enough money anyway.
As to why many RCs seem to originate over here, I think you already answered that question. For a long time RCs were flying under the radar legally speaking & I guess it just meant the world was their oyster so to speak. The big difference in comparison with the States, would be it's a lot quicker to ban anything & it's analogues. I might be mistaken but I don't think the UK particularly banned analogues / did blanket bans until Mephedrone. (I'm not sure if analogues is the term for all Cathinone derivatives? I'm not very knowledgeable about drug chemistry)
I might be mistaken with some of that, but that is my impression.