A convincing reason for not using horses for meat I found on Wikipedia, and it is about economic efficiency, rather than cultural taboo. It seems that, if you are going to use a section of grassy land to produce meat, you'll get more meat per acre by raising cattle, rather than horses.

"As horses are relatively poor converters of grass and grain to meat compared to cattle, they are not usually bred or raised specifically for their meat."
From: https://en.wikipedia.org/wiki/Horse_meat

That is similar to the reason why, historically, pork has been a favored food source with the less affluent of the world. If you're raising a meat source with limited resources, like a very small plot of land, pigs can be reared more cheaply than cattle (pound for pound - of meat - that is.)