I think the interaction with wall-jumping might be the biggest single reason. Imagine trying to mash jump up a wall if dashing shared the same button. You’d have to be super careful with the timing to not dash by accident. That’s the clearest case of the general difference that dashing is a drainable resource that needs to be timed carefully, whereas jumps are unlimited and generally not quite as important to execute perfectly.
I found the controls in Wings of Vi (with buttons for jump, flutter and attack, and has double-jump) overwhelming, but since this game doesn’t have an attack button and jumping does nothing in mid-air, it’s manageable.