Why?
I've always liked to think of Tailed Beasts like bulks of uranium. They've existed in nature and are generally harmful, but people have wanted to harness it for their own purposes. While it's possible, it's not exactly safe (the well controlled Two-Tailed host may detest otherwise), and like all weapons, require maintenance (transferring hosts when one's about to die in this case) and can be stolen.

It seems perfectly normal for certain factions to deem them undesirable to have around at all.