I think, if I'm not mistaken, Carlo Alban left because his family was in the country illegally, and they wer afraid that the exposure could garner some unwanted attention. I seem to recall somebody posting a link to an interview about that a few years ago.
I've been curious about David L. Smyrl's departure, considering there was no on-screen explanation about what happened to Mr. Handford, and all SW.org says is that he simply sold the store to Alan - because he otherwise said he enjoyed being on the show. But then again, in the 40 years book, Alan mentions that when auditioning new storekeepers for Hooper's, they were specifically looking for somebody younger, as opposed to the grandfatherly types who were auditioning, maybe that has something to do with it, I don't know.
Didn't Linda Bove mention she was simply ready to move on beyond SST?