Although Gears for Breakfast haven't given their reasons (as far as I'm aware) people have pointed out that it's likely got a lot to do with the game running on Unreal Engine 3, which does not support the Switch. A port would be a significant investment and it might turn out terrible. There could well be other reasons too, but they did state some months ago that a Switch port would be unlikely for this reason.
Rocket League is unreal 3 as well. These devs just are not good at optimization. Isn't this game 30FPS on PS4? It's not that demanding of a game visually. Sounds more like they just suck at optimizing.