I just know Toho is similar to Nintendo, it would take a big idea to get the rights.
Toho is more than just that, they distribute Universal films in theatres while they use their own Japanese company for home videos. Based on Brian Roberts, he said content and distribution are both extremely important to him. Why would he invest to keep their japanese home video company with another company distributing when he can get content and distribution by buying into Toho and also getting the rights to Godzilla, Mothra, etc.