I suspect that it is due to one of the following:
- He wants to focus those on board to the task at hand. If there's no hope of help then the only thing they can do is kill the shark.
- It's basically a macho thing that says 'I don't need anyone's help'. After all Quint is a loner who basically doesn't ask for help and expects none.
Whichever way, the story would not have been so good if the whole town had turned out to help !
