Good point. I think in the UK it stems from the perceived image of a horse: a domesticated companion, used for pleasure, sport and previously for transport. People conveniently forget that old horses mostly go to the knackers' yard, used for glue or dog food. People seem to draw the line at rearing them for food. Horses are quite uncanny readers of human emotion, probably olfactory, and that may have some bearing on the matter.
I would say, however, that horse has a very distinctive flavour. Minced in with beef or pork and seasoned it would not necessarily stand out, but on its own it is unmistakable. As is camel. I don't like camel meat.