The reason is because people absolutely hated that Tokyo map with the embankments, myself included. Rocket League is unique in that the core gameplay is so good, that messing around with it is just a bad idea. It’s like asking why there aren’t new pieces being released for Chess, or why people don’t start adding unconventional elements to a soccer field. It’s because novelties like those take away from what is already a solid experience. They have added some modes to that in non-competitive Rocket League, but RL is a unique game in that it seems like most players play the game in competitive modes, where those novelties are not appreciated, and so putting lots of work into those kinds of alternative arenas just doesn’t make a lot of sense.

I personally am glad the developers have realized they have a game like this and haven’t killed their game through adding more for the sake of more.