You can't really make the comparison to cattle and hogs. They have always been part of the food chain while horses have been part of the service side in this country. While horses occupy roughly the same plane as dogs, cattle side with chickens.
It's all about money and the cost of prepping and marketing would be high before any inroads were made in the meat industry. People won't be too happy to see horse meat next to beef, especially when the need and demand isn't there. You can't sell lean when there is little demand for bison, emu and goat. There has been too much time put into developing fat cattle and hogs and despite what the doc says it ain't changing.