As I understand it, it was mainly historical precedent, driven by the fact that most linear-algebra libraries (LAPACK etc.) are column-major (because Fortran is column-major, and hence Matlab is column-major), though it would have been possible to work around this with judicious use of transposition.
It’s been discussed before, e.g. https://groups.google.com/d/msg/julia-users/GkvDunqE0ww/2gIDmA5cAAAJ