In milongas, we move counter-clockwise because left-handers are a minority.
Ok that was the abstract, now let’s explain a bit.
We move counter-clockwise because it’s a legacy from older dances coming from older countries, like the viennese waltz. Of course saying this is only shifting the problem one inch further.
Now, why are people walking counter-clock-wise in the viennese waltz? Because when doing all these clockwise half-turns (1-2-3 to have your back to the LOD, 4-5-6 to complete your turn; and the opposite for the lady), moving counter-clockwise will make your life much easier when you arrive in a corner. Instead of a 360° turn you’ll have to execute a mere 270° . Walking clockwise on the other hand would force you to overturn each time you meet a corner.
But then, why did the waltzers of the past choose to (half-)turn to the right? They could have walked clock-wise and turned to the left instead. But not with this slight offset that puts the lady slightly to the guy’s right. The partner on your right is on the way and makes the turn to the left quite complicated. It’s no wonder that just anybody can waltz to the right, while waltzing to the left requires some technique.
All fine, an offset helps the pivots, but why to the right? Because the princes, generals, emperors, officers attending the imperial balls had not only a snazzy pomp uniform but also a snazzy pomp sword in its snazzy pomp scabbard and this sword being at their left side, it was only natural to put the lady where there was some space left, i.e to the man’s right. Sure they may not dance with the sword but they’d keep the scabbard and belt, if only to hold the trousers.
And why was the sword here and not on the other side? Well it’s the only possibility when the swords are drawn from their scabbards with the right hand, because of the length of the blade. For colts it’s different (unless the rifle is
long) and if tango had been invented by cow-boys then we might very well be walking clock-wise nowadays.
Finally, why were all these officers from the past time taking their sword with their right hand? Because most of them were right-handers, and all the armies in the world having this tendency to want their soldiers look all the same, even the unfortunate left-handers had to follow, thus wasting precious time during battles to draw their sword and dying at a younger age than their right-hander counterparts.