Same reason snakes get a bum rap in The Bible--no real reason. Animals are typically used as examples for concepts and then the superstitious take the symbolism too far.
In ancient times, sheep were prized more than goats because they had more meat on their bones than goats did, and were therefore more valuable considering their wool, too--and that's all there was to it.
The snake is a little more complicated because the snake was a widely regarded symbol of mental acuity, whereas the goat as such didn't really stand for much. It's no accident that it was a snake that presumably tempted Eve toward Knowledge, nor is it an accident that Jesus told his apostles to be as innocent as doves but as shrewd as snakes. But the goat? The goat was just a matter of being the lesser of trade value.