Several mechanisms. For one, a show running to long simply squeezes all the juice out of the premise. Popular shows are kept running too long to extract all the money out of them, but they become soulless and formulaic towards the end.

Another mechanism is flanderization, in which writers simplify characters or other aspects of the show for various reasons, mostly laziness and to conform to expectations the audience develops about how characters should act. Characters with depth become shallow shadows of their past selves as one aspect of their personality starts to dominate the others. When characters are driven by audience expectations instead of the writer's internal muse, they become soulless and predictable.

Another is overdeveloping the setting. Throwaway lines in earlier media get needlessly turned into developed storylines. It leaves less to the imagination. This is often related to the first mechanism I mention; the writers squeezing all the juice out of the earlier media.