I reckon the predominance of port-side aeroplane entry would stem from cavalry days, seeing as I have it on good authority (remember reading it somewhere...) that being a good horseman augured well for your chances of success as a pilot in the early days.
While on the subject, I'd like to put a complaint into the chief film horse-inclusion officer of Hollywood about the way those seppo cowboys get off their horses - they take their right foot out of the stirrup, swing the leg back over the horse and step down with the left foot still in the stirrup, a good recipe for getting dragged, as any fule kno.
The right way, at least as my old Dad taught me, is to remove both feet from the stirrups, slide the right leg over the horse's bum and slide down the left side with both feet thus well clear of the stirrups.