It's not "more popular" because bestiality is a fringe fetish.
It's illegal in most countries, and most humans consider it, and I quote: "morally reprehensible".
I personally don't see what's wrong with it, unless the animal in question is being forced into the act.
Note: "Fringe fetish" is the opposite of a normal fetish, (ie: anal), it's something that isn't common.