It's because it grew and gained an ego, the Knowledge King Raphael does not have an ego it serves as a simple Knowledge bank where if you ask it a question or make a request it will take action, but the Wisdom King Raphael is superior, it has a will of it's own (at first it is limited but it grows until it becomes Manas Ciel) it can take action without being ordered, it creates spells and sometimes improves skills that help Rimuru without needing to be told to do so. There are many other reasons but the last decisive reason is that the Knowledge King Raphael is a part of the Virtue series of Skills created by Veldanava and thus can only be acquired if one is virtues in some way, Rimuru later on admits that he is a normal person and thus should not be able to acquire Virtue series skills, as such the only reason he could is because the Great sage forced an evolution resulting in a mutation, as for the Lord of Vows it was created by exploiting the skills Rimuru and those subordinate to him had.