Every food commercial, advertisement, and packaging in the US is currently protein obsessed. I just bought 3 burritos, and the most noticeable thing on the entire package is its protein content. TV commercials about waffles are advertising that they have X amount of protein per serving. The products in my jobsite's vending machine have protein all over every package.
There have been a bunch of great answers already, but I'm not seeing one important factor: soy farmers. Soy is one of the cheapest sources of protein, and there's a lot of it grown, so the soy lobby is always trying to push new uses for it.