This is a very good question. We actually did consider proxy certs and name constraints certs first and had a long discussion at the IETF about these different

This is a very good question. We actually did consider proxy certs and name constraints certs first and had a long discussion at the IETF about these different options. At the end the consensus was that it would be much better to have a very minimal structure which could only do 1 thing and nothing else. DCs also have the advantage that they are cryptographically bound to a particular End entity certificate vs a particular public key only, and hence can only be used with their properties, so it really is the minimal possible thing you need and nothing more.