Why doesn't a long lasting vaccine against common cold exist yet?
Viruses that show a high mutation rate like the virus that causes the common cold escape simply from the action of vaccines against them. After a primary immune response (natural or artificially induced) against the virus in the next season of infection latest mutant resistant strains appear and the protection get with the immune response of the last season is lost. (One could say that the high mutation rate is a produce of "immunization" found by these viruses.)