I think the problem with that is it doesn't work with Flash audio. With multiprocess browsers, the Flash audio can be associated with a tab, but that's awkward/not possible if everything is in the same process.
Well, Chrome still often shares the same process for groups of tabs. I think this particular feature has something more to do with a change they made to their plugin API.
Chrome didn't have this feature for a very long time, and if anything sounded like it was actually harder because of how Chrome handles their processes.