Well, most bop kits offer the same tuning ranges as the 12.14. 20 "Downbeat" style configurations -except the bop bass drums are a couple inches smaller...so essentially you can kinda get the same tuning range -except for maybe at the extreme-extremes...which few people implement, anyway.I think it's just a style thing, really. It's cool to have one...kinda like it's cool to own a vintage sports car. Portability, yes, for sure. But yeah, they are just rarer and.....cooler! ;)
Much more than cool, there were practical reasons for urban drummers in big cities, where most of the jazz was located, to play 12/14/18 kits. As you mentioned, portability was a key factor for many drummers, including me. Try navigating subways and cabs in a busy urban setting with large drums, lot's of cymbals and heavy duty hardware.
From the late '60s through the late '70s when I was gigging in Greenwich Village, Soho, and midtown Manhattan, ease of transportation was a significant feature of a kit.
Smaller drum sizes with light weight stands, pedals and accessories, and compactness using a minimum number of cases was extremely important. I used my 20" Lugwig BD when someone was giving me a ride, but if I had to use a cab or subway it was always an 18" bop BD.
To make getting to and from a gig more tolerable, I often played a simple two piece set in those days. I used an 18" inch bass drum in a case with a pocket in which I also stored an 18" thin crash/ride and a 16" crash. Along with that I had a specially sized and built small wheeled trap case that I had made for me at Frank Ippolito's drum shop. In it I kept my Ludwig Supraphonc with my 14" hats pressed against it in a protective bag, very compact and light weight stands and pedals, and all my sticks and brushes. In addition, I had the case sized so when I turned it on its side it served as my throne with a thin circular foam pillow I made myself and stored on top of the cymbals in the BD bag.
As a result, I was able to navigate the streets, subways and cabs with just two cases, a relatively light rolling trap case, and an 18 inch bass drum case with a cymbal pocket. :)