Eh, because if there is going to be a PG-15, then PG-13 should be done away with. Then the question goes, why have a PG-15 rating when R is 17 years or older? It's unnecessary to have a PG-13, PG-15, and R. Then people will want a PG-13, PG-14, PG-15, PG-16, and R. What's the point?
Besides, the whole middle rating argument is moot. Is there seriously this significant difference in maturity between a 13 and a 15 year old? Like that article above about The Wolverine states, the issue isn't simply the rating. The issue is that the MPAA does not rate films consistently. Objectively, when a film like The Conjuring gets an R for no sex scenes, no death, no gratuitous cursing or gore, and a fantasy premise, while The Wolverine gets PG-13 for excessive violence like stabbing and the death of human beings, there is an issue. That issue is the MPAA itself.