Why aren’t the Question and Answer columns sortable in the browser?
The question and answer strings, much like the rendered cards of which they are abridged versions, are generated on the fly. Otherwise, everytime you change a template, note, noteytpe or even an add-on a large number of them would need to be updated.
That means they aren’t available to the database which does the sorting.
Oh 
Then maybe add two sort fields ?
As @Rumo said, if you make Q and A sortable, you have to sort it in Python level(low performance compared with database level), which is not feasible if you have large amount of cards(may freeze Anki).