[PKop]

It's almost like "specialization" or some sort of natural selection process. Characters accentuate specific unique aspects of themselves because otherwise they would have no reason to exist; the show could have anyone stand in to express generic qualities. Their quirks are what at first works with audiences, then writers keep going back to the well. The common aspects get selected out over time. A/B testing taken to it's logical conclusion.

[ehsankia]

I also don't think this is necessarily a bad thing. It feels like many of the examples given are the more extreme cases where it goes too far, but looking at the pilots of most TV shows (especially sitcoms), the characters are fairly generic and uninteresting, and they slowly build up their personas over time as writers write to the actor and to what works.

Community is a good example of that, all the characters definitely developed a lot, though some maybe went too far like Britta. Parks & Recreation is another one, some of the characters were actually just background extras like Retta and Jerry. The whole woodworking part of Ron also came from Nick's own background and built into the character.

[PKop]

The tropes page mentions there's a bit of distinction between writer's figuring out the character; flanderization is addressing the point after the character is basically fleshed out, then accentuating whatever they are, often past the point of caricature as time goes on. So more of a long term process. I agree it doesn't have to be bad thing, I'd go so far as to say it is inevitable to large degree. It is writers jobs simply to manage this natural/inevitable dynamic, be careful with it, and eventually end the show before it loses it's appeal. Sort of a lifecycle of "success" of writing interesting characters especially in sitcoms where personality/humor dominates story, and there will be diminishing returns as character evolves to self parody.