Well, there are five upper body segments (chest, tris, back, bis, shoulders) and only one leg segment. This fact alone would lend itself to more upper body-flavored premixes. Also, thanks to some sound information from an instructor friend of mine, I've come to the realization that often times cardio is leg work itself. Especially Cathe cardio. So many plyos, kicks, certain step moves, etc - these really tone the legs and add strength and endurance to them.
Though it isn't the same as having more leg-oriented premixes, you could always do the leg segment from Kickbox twice a week. I found that when I cut out doing 'leg' specific days and did a variety of more cardio, my legs became stronger and more defined.