I'll admit that it's stupid, that I, the electrical engineering student, would let myths of my early theatre days get the better of me and electricity is still electricity, be it for a power cord or an audio signal. It's probably never been an issue (at least for me and the people I regularly work with) been because the likelihood of messing around with the TS connectors while pumping a loud signal into the speakers is rather low.
Still, getting shocked being possible, how did 1/4" TS connectors ever make it to market for amplified signals? Especially given how most of them even have metal jackets to them. (to which now that I think back, I do remember a couple occasions getting shocked by these connectors)
Back before the days of the NL4, there weren't a whole lot of other options.
Some of the larger shell Cannon connectors were around in both three and four position, but the cost was high and they were physically large and clunky connectors. XLR's were used as I described in my previous post but were probably dropped in favor of the 1/4" TS to avoid confusion with mic level connections also coming off the stage.
You've also got to keep in mind that in the early days amplifiers were at a practical limit of 60W, voltages were lower and cable gauges were smaller. 1/4" connectors were a convenient and available connector that was fairly inexpensive.
So it became standard, and has just stuck around as power has increased and loads have gotten more demanding.