If animation can be done for the lost episodes of The Invasion, why can't the same be done for the other serials with lost episodes

Apparently sales of "The Invasion" weren't good enough to justify further work, and then Cosgrove Hall went belly-up anyway. I believe, however, that another company is presently working on a Hartnell story (I think it's "The Reign of Terror", but don't quote me on that).