Takes up a little less space than with the flute horizontal, maybe that’s the reason. Looks more genteel in the orchestra. The ladies I knew who played it, always held it horizontally.
The French method is a little different. For players who have a fleshy upper lip or a ‘labial point’ (where the upper lip has a pronounced point) it can make playing the flute difficult as we normally use the center of our lips for embouchure. Had a former student with that prob (very airy tone) and taught her to offset the ‘air hole’. It works. :)