People who want to adopt children typically must demonstrate that they would be good parents (they must be financially stable, reasonably healthy, law-abiding, and so on). This is often a very difficult process, as prospective parents are placed under intense scrutiny; and many couples who would likely make fine parents are denied.
What reason is there to regulate adoption in this way that would not apply to parenthood in general? I think most of us agree that it is a good thing that not just anyone can adopt. But why should having one's own biological children by any different? I am normally repulsed by the claim that only certain people should be allowed to breed. However, I don't see what would justify applying such demanding standards to adoptive parents but not biological ones.