Fashions come and go, and depending on how it is done is not a fashion. Some tailors put a button on as part of the seam of the leg and add a button to the cuff and people don't even know the buttons are there. It is also easy for cleaning. Seen it done another way, which I don't remember how. Having the buttons on the outside is for fashions, which are fun and how clothes have changed over time. Having two buttons on the inseam might not last and get bumped off.