Everyone simply assumes Jo/WB has been the bad guy in Potter at HHN, which ignores the fact that Universal would have likely gotten approval to do such a show for HHN before investing in personnel/costumes. After the opening of Diagon, the official A&D line about why it wasn’t used for HHN was “we haven’t asked.” But (because of the Seuss situation) everyone just assumed Jo hadn’t approved it. The reality is that, for numerous reasons, Universal Orlando/Hollywood hasn’t needed HP at HHN, nor wanted it (yet).
The original Death Eaters show in Japan occurred in conjunction with their HHN. The approval is there. But, like so many things HHN/Universal, the reason something “obvious” doesn’t happen is nuanced and less simple than a rights holder saying “No.”