Before Yuuta brought Prince Reifa into the picture, he had already planned everything in order to put a stop to the sudden invasion caused by Myrkidhr. That's why he went to the Main Ship alone, he didn't want anyone interfering with his conditions. Right now, the only person he could trust is himself and he wanted it to remain that way for this moment. He was well aware that if he were to bring Kazunari with him, he might not like the conditions he made and he might disagree, which might even cause them to fight to the death with each other.
And worst-case scenario? King Faier would join Myrkidhr in his invasion and the World of Lucid will end once and for all. Well, not like Yuuta cares about the World of Lucid being under a new jurisdiction... but then again, he had met and known a lot of people who had grown closer to him... not to mention that the Earth would be thrown into the mix.
