Because it's good ethical practice to try to work out what to do with controversial issues when the answer is not completely clear.

I see no visible controversy around the issue.

There's a reason why that German ethics council thinks it should be legal.

I don't read German except with more effort than this is worth, although I note that it begins by mentioning a case of a brother and sister having four children together and public consternation over their prison sentence. What reasons does that report give? It is that it considers the German law as it stands an unsatisfactory way to deal with the undesirability of incest, or that society should recognise incestuous relationships as equal to all others? There is a big difference. How does the report propose to handle the negative consequences of inbreeding? Or have you not read any further than ryot.org?