Because they'd rather have you using the hosted platform instead of having to support enterprise customers. There's a very steep support side when you sell something as "enterprise."
We've used gitorious as well, but due to several problems we've now switched to a GitHub organization and Gitlab. Much more happy this way in general, but the team management options for GitHub are lacking. Purely going for the Gitlab route would've probably worked out better. (We have around 50 people with Github accounts now and some of them are changing teams frequently.)