@Riot, Why can't Challenger tier duo in solo/duo queue?

I got Masters today and will be promoted to challenger tonight. My friend just told me after I get promoted I can't duo with him (or anyone) anymore, I checked the ranked play FAQ and he's right: Challenger tier players can't duo with anyone.

There's no explanation on the FAQ page at all... why is that? Kind of lame tbh. I honestly think you should be able to duo with other Master or Challenger players at the very least, maybe Diamond 1 as well.