Why do some infectious diseases not have vaccines to prevent/cure them?

Time,money,difficult: There are ongoing efforts to develop more vaccines but the process takes time, researchers with training & interest & lots of money. Once a potential vaccine has been developed, it takes years of testing to get to the point that testing on humans is permitted. Then it takes time to collect effectiveness & safety data to get it approved by the fda. The process is long but one that assures safety.

See below: If multiple viruses are involved as causative organism, making a vaccine against them is virtually not feasible. Example of common cold. Some inactivated virus, e.g., RSV or serotypes of bacteria, e.g., meningococcus, are simply not immunogenic to produce adequate antibody response.