There are good and bad reasons for the vast diversity in the Hawaiian Kenpo systems (totally differant from Japanese/Okinawan Kempo).
In contrast to the strick traditional arts like Shotokan, Goju Ryu, Wado Ryu, Shorin Ryu, Aikido etc., Kenpo evolves constantly.
The strick traditional arts tend to stay with what they "were always taught". In Hawaii there is a vast mix of cultures that is rarely found anywhere in the world. Japanese, Okinawan, Korean, Chinese, Filipino, Samoan, Hawaiian, Europeans, Indonesians, and others are all represented on the islands. So there is a opportunity to learn fighting techniques from all over the world.
The origin of James Mitose's "Kenpo Juijutsu" is something that will probably always be a mystery. But close examination of his teachings and even the name of his system tends to point out that it may be a blend of a jujitsu system and a kenpo or karate system. So, in actuality, he may have started the evolution of Kenpo.
We do know that William Chow studied other arts such as boxing, judo, jujitsu, and kung fu. He added some of these techniques to his "Kara-Ho" system. And, of course Sijo Emperado's Kajukenbo system is the first example of "mixed martial arts" in America, perhaps the world.
Americans have always looked at things and tried to figure out how to make it better, or at least make it more American.
And you have to remember that James Mitose only taught in Hawaii for about 10 years. He never really defined his art to his students. At times he called it "Kenpo Juijutsu", "Shorinji Kenpo", "Kosho Ryu Kenpo", and "Kosho Shorei Ryu Kenpo". His first black belt, Thomas Young told me that he had never known Mitose to use the name "Kosho Ryu' or "Kosho Shorei". He (Young) first found out about the name from reading Mitose's book, after Mitose moved to California.
So what I'm trying to say is that Kenpo never really had any ancient traditions to adhere to.
So people like Prof. Chow and Sijo Emperado looked at Kenpo and thought about how to make it more effective as a self defense system that fit their times. Sijo's attitude was that he wanted to see "what the other guys had, and if it was good, we'd use it". So, as Kenpo and Kajukenbo instructors spread across the country they supplemented their knowledge with techniques from martial artists in their areas.
Evolution and adaptibility are the main reasons for the variations in Kenpo systems. In strict traditional systems there is no room for evolution. You do the techniques you do because it "has always been done that way".
Take Aikido as a example. Many of the attacks in Aikido are done in a downward chopping motion simulating a downward sword strike. Weapons defenses are also done against a bokken (wooden sword). Now in Kajukenbo (Original Method) weapons defenses are done against a knife and club. There are no weapons katas with any of the ancient swords, spears, staffs, etc. So Kajukenbo has adapted itself to the American/European society, where you would most likely be attacked by someone with a knife or club type weapon. There are many other examples of how Kajukenbo and Kenpo have evolved to fit the needs of modern societies.
As I said in the beginning, there are also bad reasons for the vast differences amonst some of the Kenpo systems. This comes from people wanting to break away from their instructors, and others who want to be grandmasters of their own systems. Some of these people and their systems have become outstanding contributions to the martial arts, and some have become total discraces to the arts. You can now buy Kenpo black belts on Ebay, and there is at least one Kenpo instructor who has a 1 year video black black program.
