Bethesda Softworks owners of the Fallout license have sued former owners Interplay over breaching a licensing contract between the two parties. Interplay did not, according to the legal filing, consult Bethesda over packaging, advertising, and promotional material prior to the release of Fallout Trilogy (Fallout, Fallout 2 and Fallout Tactics). Given that Bethesda had Fallout 3 out on the shelves we are pretty sure they weren't too happy about the naming of the product.
Furthermore Bethesda have issues with Interplay distributing Fallout titles across digital platforms such as Steam. But the main issue is that Bethesda are looking to terminate Interplay's license to make a MMO in the Fallout universe. There were a couple of conditions, including raising enough funds to pull it off, that Bethesda feels Interplay have failed to live up to. I guess the future of Project V13 is in the court's hands now.