This came about at a time when the merchant class was still evolving out of the peasant class. Peasants had no time for graceful divisions of labor. If you wanted to eat, you worked. At a time when life was often short, brutish, and nasty (thanks Hobbes), it was in some ways easier to bend the rules on things like gender restrictions than in later, more outwardly civil times.
In fact, a lot less things were divided simply because there was no easy way to do so. For instance, most medieval women rode astride rather than sidesaddle, because the only sidesaddles available were precarious to sit in and required a person to lead the horse forward on foot. This was frequently impractical even for the nobility. It wasn't until the 16th century that a sidesaddle with a pommel was invented, which made it a lot easier to control the horse while remaining seated.
|An illustration from the Manesse Codex, circa 1340|