The short answer is that MD5 is widely discussed because it's had widespread historical use, and MD6 isn't because it's relatively unused and has faded into obscurity due to a multitude of better of alternatives.
Why isn't it more popular? It was an entrant in the SHA-3 competition, but was eliminated in the first round because it simply didn't stack up against the competition. It was noticeably slower than other competitors, didn't offer much in the way of advantages over them, and its formal proofs weren't complete.
In the meantime, SHA-2 is still considered secure and is in widespread use, the Keccak digest was chosen to become SHA-3 and serves as an emergency backup should Merkle–Damgård constructions in general become broken, and BLAKE2 — a derivative of one of the other finalists in the SHA-3 competition — has gained support as a direct successor to SHA-2 due to improved speed, security, and features. Given that, there's not much of a reason for anyone to further consider MD6.