The holographic doctor was not meant to be used long term. It was made in the hopes of never being used. You would only need it if your own medical team was gone, and in that case you would return to base and get a new team. Voyager couldn't do that so they had to have the EMH running long term, something the program wasn't meant for.
But the program was still quite massive. It combined the knowledge of 47 different doctors, and something like 5,000 different medical procedures. The program was too big for more than one copy.
The cardassian consultant on the other hand was a simple program based on one person's research. It wasn't a backup fully functioning doctor meant to deal with any medical crisis that may arise. It was a talking encyclopedia. A much simpler and much smaller program