Perhaps simply because it's the most effective way of teaching! 

In life, we should always try new things and new methods, teaching included, but we should never implement change for change sake. 

If a system ain't broke. It don't need fixing! but never stop trying to stay on a course of continual improvement. 

The key point in education is the teacher and how they get across their message to their pupils. The teacher is the key to learning anything in life and all they really need are their mind, personality and voice to get a message across. 

Sometimes, simple is best!

Interesting question, thank you :-)