A good, but difficult question. I think it may have to do with some of the immune evasion genes found in the virus and the fact that the infection is primarily a mucosal infection. These infections frequently do not form long lasting immunity. Rotavirus and Respiratory syncytial virus are two examples.
If genetic drift and shift occurs, this would lead to genetic diversity and probably antigenic sin, hoskins effect.
Second is the antibodies may diasappear but the memory cells can respond earlier if bofy is exposed to virus again.. but the same type or related structure