what's the difference, though, if there's room for all those OTHER holo-characters, at least one of which was a doctor, complete with skills. Wouldn't work from a TV standpoint, but if they had the Cardassian in there for 'historical' purposes, no reason that dialing up McCoy would have been any harder. Just can't film it.
And books later used this easily, but there shouldn't have been anything special about the program for the Doctor. If there's space left on Voyager's hard drive, no reason you couldn't just copy it and make more, or run more than one copy at once. Again, no good for TV, but it would be a retardedly-simple "real world" concept for Trek. If they imagined a situation where they needed a holo-doctor, they could just as easily look at that and imagine a mass-casualty event requiring an extra half-dozen doctors.
Only reason for the single, un-copyable version of the Doctor was so that they could use it as a plot point here and there when they felt like ignoring other things they've shown. That and to make you actually care about the Doctor as a 'person' rather than how you care about the helm console. And ease of filming, I suppose...