Because if there are multiple inverted units, you'd need to add parentheses to put them all in the denominator. Tidily giving each a ^-1 is clearer (especially with superscript exponents).
You can use superscript Unicode characters on HN: km·h⁻¹.
Ah but you see, I am on my phone and very lazy.
What's wrong with a/b/c
I guess they stick rigidly to things that are formally defined, in case somebody thinks that division is right-associative and it leads to the Challenger shuttle exploding again.