I don't think you are appreciating the complexity of the reward, incentive and moderation, curation system in Stack Exchange. It's really non-trivial. It's not a simple thread structure, it's a delicate balance.

But since it's already developed, why not use the open source code of Stack Exchange and implement a third party app on top of Steem? That would be way more feasible them trying to do technical q&a inside Steemit.

I think applications like this will take off as soon as the core dev team clarify what opportunities there are for revenue generation for the application itself. Without that clarification nobody serious will invest time into building other applications on top of Steem because the competition is too steep for another Freemium company.