Considering the data that has come out basically saying that people who have gained natural immunity (they got sick and built up B lymphocytes of which, some will remain as memory cells) have it last a few months is disheartening. That is likely the reason that multiple doses would be necessary. The more the immune system is exposed to an antigen (some sort of protein that evokes an immune response), the more memory cells are saved and the more specific they become. I don't think it's for the money, it's to make sure the vaccine is actually effective.

All of that goes out the window though if the virus mutates enough that the vaccine is no longer protective. It will be a fine balance between getting people to get the vaccine and getting enough of the population immunized actually protected (since vaccines aren't magical things that automatically protect you, there is a certain percent of people who will get the vaccine and won't develop a protective or adequate response, which is why we need HEEEEERRRRD IMMUNITY).

Edit: I didn't really discuss memory T cells, but they are also important in protecting against a virus as they will be the ones killing cells that are infected with the virus. Same issue still applies though: you need to get enough of them to stick around long enough to give protection.