I asked this a few times, but no one has ever given me a satisfactory answer. My guess is it follows the tradition of lights in the old days when you need to untwist to disconnect. :shrug:
I think that's the main reason -- the simplest methods of breaking a circuit involve moving the two ends apart, not together. The problem with a Mag-style (i.e. minimag or solitaire) twisty is that it takes more length. In fact, it takes up just about as much length as a clicky, but most people seem to prefer twisties as keychain lights
because they're shorter. It's a no-brainer, then, that it'll be hard to compete by selling a bigger light that costs more to make.
I suspect there's also a slight reliability penalty for loosen-on twisties (given the same level of dust intrusion), inasmuch as you can't readily guarantee twisting to self-clean the contact surfaces. A tighten-on also allows you to more readily clean all contact surfaces yourself. With those, the loosen-on's advantage for storage in dirty environments doesn't strike me as a clear win.
But I do prefer the way loosen-on lights handle -- I can crank down the head as securely as needed to prevent accidental loosening, but once it's cracked open, I don't have a heavily-compressed o-ring stiffening it up, so I can twist it on and off freely.
