Money.
If you imagine going to Disney with an extended group of a large family, you would want pictures. So each little group wil have their own time together, and would want to take pictures on their own, but you'd also want group pictures too. At the end of the trip, if you order the photopass CD, you'd want to make sure every smaller family group got access to it. So perhaps you'd all go in on it together, instead of one person/smaller family group buying it on their own.
Thats the concept.
It was a Share that got me to realize what a great deal the photopass CD is, actually. I don't share anymore, unless it actually is with family (like our future WDW trip with cousins and aunt) or a group of friends like the ladies trip (though actually I didn't partake in that CD, but I posed for pix with them) where I'm actually there at the same time WITH those people (even if not attached at the hip to them the whole trip), but I will always be grateful to the disboards share that made me see how wonderful the CD is!