Intellectual property rights. Businesses will hold on to those until death. There are way too many titles that could be profitable, but because of a license issue here or there (Baldur's Gate, Tales of the Unknown: The Bards Tale for examples). It's like the one going on between Fox and Marvel right now. Last I heard, Marvel was going to kill off all the X-Men and is ending the Fantastic Four comic because Fox refuses return or at least negotiate a decent use contract.
It is such a sad thing to... so much could be done with the Ultima IP... EA is just to stupid to put it to use.