Sword of Spirit
Legend
If I had to guess, it's because the discrete move-type action in 3E played well with the action economy. One of the side-effects of the Next single "action" system is that a lot of things just aren't actions, or happen alongside an action; a return to Standard action and Move action would "solve" a lot of that weirdness.
While I agree that they have been (unfortunately) edging towards reintroducing complexity via the current system, I don't think move actions is a solution.
One of the things I found so refreshing about 5e in the early packets is how you didn't have to worry about why type of action something was. It was either an action or you could do it free. Movement was its own separate thing, and didn't have to be taken account of.
They've decided it needs to be a bit more complex than that, since there are spells and special abilities they want you to be able to use without taking your whole action. By the time they're done, we might end up having an action economy of Action/Bonus Action/Part of Action. I certainly don't want to allow movement to be used as another type of action.