Why should they? Those games are enjoyable for some people, and they are story-oriented, so the gameplay isn't important there. What matters is if the stories are good, and in my opinion they are (at least for The Walking Dead Season 1 and The Wolf Among Us, which are the ones in this format I've played).They've done little to no change in their most recent games. They should at least make an attempt to try something new. But then again it's TellTale and they get a freepass for it. If this was any other developer people would be more critical of them for following the same cookie cutter formula.
Change for the sake of change is not necessarily a good thing. I really like the formula, and I don't want to see it changed unless it's necessary to tell a better story. As long as it isn't, I'll keep enjoying these games, provided the writing is still good and engaging.