He believes that oration is an art form, and because of such he believes that the way many sophists taught the art of oration was wrong. They applied far too many rules and regulations, in an attempt to make oration more of a science. Isocrates didn't like this. He was a strong believer in kairos, or fitness for the situation. This means that there are no hard and fast rules that can be applied to oration for every situation is unique and as such every situation requires a completely different approach. This plays strongly into another one of Isocrates beliefs, that oration is heavily reliant on natural ability. One can be instructed by another in the art of oration but unless they have the god given ability it will all be for naught. He also distanced himself from other sophists of in the way he taught. He didn't travel around teaching, rather he stayed in one place in order to better tutor his pupils for years at a time.
I think that Isocrates cared more about the actual art of oration than other sophists, who I think cared more about the business of teaching for profit. Part of the reason that the sophists tried to apply so many rules and guidelines to oration is that it makes it easier to teach to more people.