Do you think Ben Kenobi should have started teaching Luke about the Force when he was a small boy? It was only by chance that Luke was introduced to Kenobi. Don't you think that Yoda and Obi-Wan should have been more participating in the events?

Actually, in Revenge of The Sith, Obi-Wan took Luke to Tatooine to watch over him and keep him safe from Darth Vader, not necessarily to train him. With this in mind, it is reasonable to think that Ben Kenobi knew that Luke was in trouble with the Tuskan Raiders, either by watching him or through the Force. Also in Revenge of The Sith, Master Yoda exiled himself to Dagobah after he couldn't defeat Darth Sideous. He had no way to get involved until Luke came to him.