12s generally makes sense as a battery layout with batteries in a “U” shape configurations, efficiently using space. Most/many decks wouldn’t easily fit two more p-groups in that U configuation.
18s allows you to use space with similar efficiency and layout the p-groups in a “N” configuration, or a “U, U, U” configuration like lacroix does.
Mostly though, every reason someone would go 14s also supports going to 18s, so why would you settle for 14s.