The fact that is has been approximately constant over such a long period of time is what I find most surprising. I would naively expect many of the improving factors that you mentioned would lead to an increase in free throw percentages as well. Clearly, there are some teams which hit >75% and even occasionally close to 80%. I like the idea that there is a tradeoff in that every hour spent training for free throws leads to a decline in other skills. But still what's special about the 69% number? I would like to see how much time the 75% teams spend doing free throw drills compared to their competitors.
Similar to optimisation theory, each skill has positive and negative effects on other skills. This has been thoroughly explored in fields such as profit maximisation and breeding, but little has been done in sports.
