Honestly? I think it's because column vectors are less of a pain to write on the blackboard. I'm being serious. There is no reason to say that columns are vec

Honestly? I think it's because column vectors are less of a pain to write on the blackboard. I'm being serious. There is no reason to say that columns are vectors and rows are linear functionals. The other way around is a perfectly valid convention. But I did find that when I taught linear algebra, column vectors were just easier to write on the board.