This is a common answer, but the only definite answer is an answer from Game Freak themselves. However, there are a few likely reasons that we can pretty much guarantee are part of their reasoning for changing the list.
- Keeping the same TM list would be incredibly boring and would limit the options Pokemon have in terms of moves. The amount of moves increases each generation, so changing the list of TMs would allow older Pokemon to learn newer moves.
- Game Freak sometimes want a large amount of Pokemon to learn certain moves. This is evident with moves like Hidden Power and Toxic which have both been TMs since they debuted. So that means they can replace older TM moves with newer ones that they want a variety of Pokemon to use.
It is pretty much about making the newer moves that Game Freak want Pokemon to learn available to players and keeping the game interesting by changing a few moves around.
I assume the moves you believe are useless are Ally Switch and Quash? Not necessarily. The former can be used to change positions with another Pokemon in doubles and triples, so it can be useful if you can swap a Pokemon in that resists a predicted move, and the latter can allow various methods of setup while guaranteeing the Pokemon affected by Quash does not prevent it from doing so. Game Freak makes some moves specifically for use in doubles and triples, and those moves are examples of that. The only useless move is Splash really. Some moves are better than others obviously, but they all have a use of some sort except for Splash which was an intentional gimmick and joke in the game.