One simple possibility is the disconnect between the media and real life. Real life, for the most part, is rather repetitive and dull. This is a death sentence for most shows, who have to be continually funny and never be repetitive. So when they go through all of the possible jokes for a character as they are, they slowly exaggerate the character to give them more material. When they've used all the jokes they can for a slightly dim character, they slowly stretch them out to a Too Dumb to Live character.
This can become very annoying if the character's aspects were originally unique and subtle, only to become more stereotypical as the story progresses, to fit the requirements of more clichéd plots. Especially dangerous if executives think that doing this will appeal with their demographic better and boost ratings. When it happens to a character favored by the fanbase, it will often be cited as the moment the show jumped the shark. Not every viewer likes it when a character becomes a parody of themself.
However, Flanderization is not necessarily a bad thing. In some cases, viewers may find the over-the-top version of a character more entertaining than their original, subtler version, and can also be a result of the author having more creative enthusiasm for the distilled version. There has been serious study on why this works with audiences. Philosopher Henri Bergson in his essay "Laughter" wrote that comedy is based on inflexible behaviour, i.e. living people acting mechanically in their trademark manner, however inappropriate the circumstances. Early examples: Malvolio (from William Shakespeare's Twelfth Night) will always be self-important and anal-retentive and is thus easily gulled. Jack Benny will always be stingy, even with a gun to his head.
One possible reason for this trope that doesn't devolve into Viewers Are Morons is when The Artifact gets flanderized. Suppose there is that character who just kind of tags along because the writers no longer can think of anything interesting to do with him, but his popularity means they can't just get rid of him. Reducing his screen time is only half of a solution; if he's just stuck with nothing to do, he may fall into the danger of being seen as The Generic Guy. One possible solution is to overplay the traits that made him popular in the first place, so he can at least spice up his appearances with his trademark jokes and catchphrases.
Another possible reason for this to happen, especially on, say, a long-running show or collaborative work, is the almost inevitable changeover in writers, complete with different priorities, different points of view, and different interpretations of the characters.