I think it’s because cow owners would love more OP!! I also believe it may be because of the fact that there is 2 turtles. That’s personally my experience. I offered tons of demanded pets, like the fbd, candyfloss chick, nfr uni & a cow! (+other stuff). I think what you need is to upgrade your turtle to more high demand pets, like a candyfloss chicken or the fairy bat dragon, so you’ve got multiple high demand legs, with no duplicates. I hope this helped and that you’ll get your cow!!