Why is this teen being charged with manslaughter instead of murder?

That article does NOT say the cops identified the person who did the stabbing. You might want to read it again. There is a corrolorary to the felony murder rule in some states. The misdemeanor manslaughter rule, where applicable, would allow for a manslaughter conviction if some dies during the commission of a misdemeanor, even if the death was not intended or even if it was accidental. When you do not know who did the stabbing, then this is a place to start charging until the investigation is over.