I just think echo is too specific as opposed to sound. Echos are just one very small aspect of sound as a whole. When it comes to the game, a move like screech and uproar both pertain to sound as a whole. But they have little to do with echos. So, if they created a type called echo type, it just sounds ridiculous to call those moves "echo" based. But then again, this would be coming from the company that chose to use grass type instead of plant
You could say the same same thing with Fire and heat, and Ice and cold, and Grass and plant.