Interplay failed. In a whole lot of ways. They sold the rights to Bethesda (IE, they were paid, by Bethesda with an agreement saying "We won't make anymore Fallout games, and you can do what you want with the license."). They did that because as far as Interplay was concerned, Fallout was dead weight that they weren't doing anything with.
Bethesda goes on to say "Ok Interplay, we're not particularly interested in making a Multiplayer Fallout Game, so give us some money, and if you can prove to us that you can actually make a Fallout MMO, then you can make it." Interplay sits on that, doesn't do much with it. Then Bethesda makes one of the most successful games of the year with the Fallout Franchise.
Suddenly Interplay decides "Shit! Fallout is Hot again! Let's get this MMO cracking!".. They start getting cracking on the MMO, but they haven't put any real work on it, and the clock for their contract with Bethesda is counting down.
They bark up "Give us more time!" Bethesda says "Alright, you have a few more months to prove to us that you can bring a viable Fallout MMO to the market." A few months pass, and I'm sure Interplay is working hard to meet that demand. But they can't. They bring it to Bethesda and Bethesda says "What is this shit? Your time is up. The Contract's Null and Void."
A week or so ago, Interplay announces plans to covertly (lol) produce and they're holding out hopes, release, a Fallout MMO.
Bethesda steps up, cries foul, and sues.
Fallout 1/2 fans step up and say "Nice fucking job Bethesda, let the company make the games they invented, greedy you asspricks!"