Why do so many different people have so many different ways of training?

Here are some reasons:

1. Ego. It's a big reason, and it gets in the way of truth. I'm not referring to a healthy ego, but egotism.
2. Charisma. The ability to influence. This can undermine our intelligence.
3. Laziness. Because human beings prefer that little magic pill (or technique) instead of actually breaking a sweat through the effort of exercise.
4. Lack of knowledge.
5. Misunderstanding of truth.

Understanding Truth

I am often astounded seeing other people interpret physics inaccurately. There is often a desire to “bend” the physics of a situation to accommodate a particular technique favored by the instructor or student.

Truth doesn’t bend.

The physics of a body with four limbs, having a range of mobility and specific energy consumption, does not change because a person desires it. 

A simple example is:

A straight punch down someone's centerline will always be faster than a hooking punch. 

Why?

1. There is more muscle recruitment for the hook. More muscles involved for any action = less efficiency. More energy reserves are tapped for the effort. More effort is required for the movement. 
2.  A straight punch's momentum is carried more efficiently toward its intended target. A hook's force is reduced during the arc of its path. For the same effort, the opponent will feel more energy transfer into their body from a straight punch than a hook.

Some human beings are capable of great extremes above the norm. But even they must function within the limits of the effects of light and gravity, inertia, momentum, force and acceleration, surface tension, leverage principles, etc.

The Terrible Truth of Laziness

I once watched an instructor define a Muay Thai, rear round kick for a student.

This particular kick was of his own invention. 

He detailed that one should keep the supporting leg completely still and use the "springiness of the tendons" in the support knee to help the student rebound from the kick after firing it.

This person had no understanding of the amount of stress the tendons and their muscle connections were under during that repeated motion. 

Tendons aren’t springs. They are dense, connective tissue with little flexibility and blood flow that help connect a muscle to bone. They provide a structure for the muscle attachment.

The instructor I mention fell into a trap most other people do: laziness.

When first learning and ingraining a proper Thai kick, it requires more effort to step into the line of attack, rotate on the ball of the foot, bend the knee slightly, and fire the kick. You have to work balance and adaptive mobility.

But the positive side of this effort is that you’re less likely to blow out your knee using the proper method during the stress of a real fight.

You will have far more power and penetration into—and through—the target.