The impression I have from period reporting is that American carriers still saw voice and SMS as core businesses and viewed anything that directly tried to tran

The impression I have from period reporting is that American carriers still saw voice and SMS as core businesses and viewed anything that directly tried to transition users away from that as a threat. Including a messaging app on a phone wasn't dangerous if both parties had to have it, but a seamless upgrade from SMS as the default would lead to nobody using SMS. iMessage was exactly that sort of threat, and articles from the time mention carriers disliking it. Not selling iPhones would have hurt a carrier more than it would have hurt Apple, so their negotiating position was weak. Google didn't have as tight control over Android as it does now, so carriers could pressure OEMs to not include Hangouts, or at least not make it the default SMS client.