There is so much because of rule 34. Allow me to explain. You have a favorite anime, cartoon, book, whatever. Rule 34 states that, because it exists, there is porn of it. I'm sure you already knew this. Ok, let's carry that thought. Furry is a genre more than an individual entity. There is an abundance of furry/anthropomorphic work, both mainstream (such as Zootopia and Star Fox) and individual (such as a particular furry artist. Apply rule 34 to each individually, and it starts to add up. Saying furry porn has a greater quantity than porn for any specific work (say, MLP, to give an example with an abundant amount of porn) is unfair, since furry covers a much broader range. A more fair comparison would be to compare quantities of furry porn and anime porn.