It just seems to happen that in many pairs of opposites, one of the two is the 'default' adjective. It's possible to use the other, but only in particular contexts - for example:
A. He's the shortest man I've ever seen.
B. How short is he, then?
Note that it is not just in wh- questions that we use the default form - He is six foot/1m 83
Mary is three years