They were, the problem were that the protocol wasn't very good, and that they were closed source so there was a single point at which to cut them off - at the point of distribution. It seems to me that the same reasoning that was used in the lawsuits against Kazaa (which ultimately caused Kazaa's demise) could be applied to this product. Time will tell I guess, when it gets big enough to target - for now the centralized torrent trackers are a lot easier to use and more widely known. Tribler being open source might make it harder to suppress with legal tools. I think the logical step (if it would become mainstream) would be to sue uploaders individually.