Well, I carry a Colt Lightweight Commander in .38 Super. I carry it because the 1911 has an excellent trigger, a low bore axis, is very slim for its size and enjoys phenomenal support. A shorter slide removes the Gov't Model pinch, and an aluminum frame makes it much easier to tote around on a long term basis. Oh wait, that's all platform. Drat.
Maybe it's because .38 Super is much lighter kicking than .45 ACP, which is helpful when you go around shaving whole pounds off the weight of the launcher. Also, it has an OAL that's actually designed to feed in a 1911, unlike 9mm Para, yet it still gets to piggyback on all the excellent advances in bullet design developed for the shorter, more popular round. But dangit, those are all caliber reasons, do they don't count either.
Hmm. Maybe because it's Super effective. Says so right on the box. But then that has to do with terminal ballistics, and thus isn't a valid answer either.
I guess the only real answer to your question as asked would be "Heck, I don't know. No reason."
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