Well for me, it's because although they're cheap and dedicated, servers, they aren't particularly good machines and the support is pretty much what you'd expect at that price (very basic).

That being said, I do think they're great value, and I have a reverse bidding dedicated server from Hetzner, that I use as a test and staging server. But I wouldn't dream of using them for production instances though - for the amount you'd have to pay to beef up support and get access to things like out of band console access, you might as well pay for a server from Bytemark.