the Great Filter of private communication

we have the maths! we have the technology! ... yet actual robust, private communications remain elusive. where the "Great Filter" thwarting our privacy codes? is it usability; anything more than invisibly automatic a failure? is it cost; anything more than zero too much to bear in the market? is it correctness; anything less than a single mode always secure, broken? perhaps all of these above, each a requisite element of robustness, further compounding the difficulty of realizing an ideal.