Because energy bolt one, two, and two; and protection one, two, and one make no sense? To me those names are so boring and mechanical/sci fi.
However the reason they didn't do that is most spells can be cast in higher level slots, so you don't need number versions. In 5E you have magic missile 1, 2, 3, etc. Different spells are different, just not upscaled of previous spells levels
Also I think I don't like separate spell lists. That's just me, it's not a criticism of 5e.