Pure guess, but given the high market share of iPhones in the US in that demographic, it may actually be the higher usage of iMessage among young adults that prevented the ascendancy of WhatsApp there.

I think this is all stupid. One the one hand the tech companies get slammed for not inter-operating, then Apple gets slammed for inter-operating. Loads of phones have come with vendor specific messaging apps and such, it's just that Apple committed the crime of making theirs decent and successful.