If you read the post again, you'll find that your question (why Blizzard doesn't build tutorials to educate players on proper threat management) has already been answered:

The current threat system isn't fun.

As unfun as the current system is, how much worse would it be to have to go to school to learn the unfun system before you could use it? Why would Blizzard spend resources developing an unfun training module for an unfun system, when they could spend those resources on making the system fun?

Remember, developing a more compelling gameplay mechanic doesn't change the fact that for months or years, you understood the system and differentiated yourself as a good player because of it. You will still be able to differentiate yourself from lesser players in the future. Complaining that all players should continue to languish under an unfun mechanic just because that's the way it's always been, is just cognitive dissonance.

For the record, "cognitive dissonance" is never a good reason to maintain the status quo.