Roy wasn't stupid, and neither was he an airhead that thought that the whole world would follow behind him, once he asked them to.
In fact, even Sara and Sura weren't willing to serve him despite him becoming their Master, which meant that the chances of the rest doing so were even slimmer.
But he didn't care, it wasn't like he needed the help of the whole world to reach his goals anyway. Even the power he was trying to build was only to keep the flies away while he took care of the main dish.
So, he had no intention of trying to recruit the rest of the beasts. Well, if something happened, and he managed to do so, it wouldn't be bad, but would something like that happen?
He wasn't some character in some anime or manga that would talk about friendship, pretend to be a hero, and make the whole world his friend.
Even if someone were to propose that to him, he would still refuse. Being a hero wasn't that magical and good as everyone thought.
