Because Potara fusion makes you fuse Genitals first and we all know you have to also have the same genitals. Kaioshin and Kibito dont have D's so they fused together. When Goku and Vegeta fused together you can see it making them fuse D first. Same with Black and Zamasu. This is one of the main reasons Vegeta does not like to fuse. It makes him too curious.