Actually pretty nice of Byakuren to proselytize in the Beastrealm. The way I understand it, being reincarnated into a beast is bad in that you're totally driven by instinct and would be too fixated on survival to even consider the teachings of the Buddha.
I think the general idea is that the Humanrealm is the most ideal realm to achieve enlightenment. You're too torturous as a being in Hell. You're too hungry as a Preta. You're too dumb as a Beast. You're too... emotional(?) as an Asura. And you're too happy as a Deva.
