I suspect it's a marketing strategy. A lot of people have bought into the idea that they have to use conditioner from the same line as the shampoo "because they're meant to work together." Therefore, if you run out of conditioner before you run out of shampoo (or vice versa), you're more likely to buy the same brand and line again.
Peace,
Morgan

Baby Fine 3B, low porosity, normal density and elasticity
CGing since July 2008