1) The producers were too lazy to write for more recurring characters than they absolutely had to.
2) They were more interested in paying for explosion-y effect shots than speaking roles.
3) After seven years of the EMH, who literally didn't need to sleep or rest, they flat-out forgot
that a ship should have more than one person in sickbay.
4) Some combination of the above.
There's no question that there should have been visible nurses/medics/what-have-you. A missed opportunity, like the missing Neb crew in the Matrix
sequels. We get, what, a complement of eight in the first movie, then just four