I asked about this specifically and the company responded that it was because the back is higher and they don't feel it's safe for a younger baby to be below the top edge of the back. (Did that make sense?-not sure how they said it but I think you get the idea)
Anyway, I think they are afraid of a lawsuit that a baby was smothered because it's head was covered or something. That's my guess.
Of course, we that use mei tais know this is not the case.
I thought about exchanging mine for the taller one but I'm gonna keep it since it works fine for now. But if I was buying a new one I'd get the toddler one for sure!