I think that the truth is that no one does. People eat certain animals out of custom. Intelligence really seems to play no factor in the actual logic of the ethics that these people claim to be supporting.
Anyway, I added an interesting new argument to the common arguments section. I get a bit saucy, but it really is a dumb argument. Anyway, go have a look and see what you think. This particular argument is on eating or not eating certain animals based on intelligence as a meaningful criterion for making that decision.
Have a great day.