I think the reason his popularity waned was that his slapstick humour was very peurile. As a young person that is quite endearing but as he got older it was almost like he was retarded and that was not funny. It would be like laughing at a disabled person falling over.
You could be talking about Jim Carrey or Adam Sandler or any of a number of other 'comedians' whose humour has tanked their careers. At least Jerry's humour was 'childish innocence' and not sophomoric like Sandler et al.

