1 )Not everyone can get vaccinated due to existing pathologies or simply being allergic to some components of the vaccine. For these people it's important to reach herd immunity as it's probably their only defense. Consequently, if people decide not to get vaccinated, reaching that threshold is harder and harder and it will force those who can't get the vaccine to live in segregation/fear.
2) The cost of a vaccine shot (both in terms of money and equipment/personnel needed) is orders of magnitude less than curing the disease. If you don't get vaccinated and you get said disease more healthcare resources need to be allocated to you. This will probably lead to increase taxes or budget cuts in other areas or, more worryingly, you could indirectly kill someone else with a curable disease because there were no resources to cure him/her.