Mark;
I can't say for certain, but I think the reason most re cylinder is just a matter of their time frame. As has been pointed out the pin-fire system was relatively short-lived & most were made before choke boring was fully developed & in general use.

According to Mr Crawford mine was likely built in 1863-65 & it is late for a pin-fire.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra