I believe lots of Welles' stuff used to be pretty stubbornly (and moronically) protected by the Welles estate or whatever, making many of his films difficult to distribute in newfangled formats like DVD (even though they usually found their way onto some seedy Spanish R2 versions, as in the case of Ambersons and Othello). I suppose the people who control them have softened up in recent years, what with most of his stuff now being available, but I have a feeling that Ambersons and possibly Othello are among those still considered "problematic". Otherwise we'd surely have seen Criterion releases of them by now. There might also be issues with prints, etc.
