OVH facility, Roubaix France --- why are so many seed boxes all in one location?

Just curious.

What is it about the OVH facility in Roubaix, France, that makes it such an attractive place to host a seed box?

It's a self-feeding circle. They offer cheap 100 Mbit servers with a decent amount of storage, so people started using them for seedboxes. Since transferring data over OVH's internal network is faster (and cheaper) than sending it across the internet, more people use seedboxes there so that they'd get better speeds to other seedboxes.

The more seedboxes there are hosted at OVH, the more people are going to want their box there. Having hundreds of terabytes of data and thousands of peers just 2 or 3 milliseconds away is very nice when your goal is moving the most data in the shortest period of time.

Basically what Vars said, with some additions.

Europe is the place to be for internet connectivity, the rest of the world bar Japan just can't compete, due to size and population density and shitty government policy (shakes fist at AU government).

The other part of it is the illusion of performance. They do offer a reasonable service, but its nowhere near as solid as the other providers, even server.lu tends to be more reliable. OVH has huge peering issues (past 2 weeks slow FTP/HTTP to a lot of Asia/AU), which tends to limit people's options. But when it appears 50% cheaper, you still go with them, because at the end of the day your buffer/ratio will still go up :)