The one in-universe criterion would be that these tubes should be as difficult for the crew to access in-flight as possible, to hide their true nature. This might indeed best be achieved by having 72 individual single-shot launchers that come pre-loaded (or at least starbase-loaded-by-stevedores-in-black) and don't requite any in-flight tampering such as loading of the next round...
On the other hand, these were supposedly refitted in a somewhat hasty and clandestine manner, and adding 72 tubes might be beyond the means of the small cabal of warmongers. I mean, if an important man comes and says "I want this single secret device installed, no questions asked, and it needs these power leads and this control cable", you do it; it's simple enough to plug in yet another module of weirdness to an already flexible design. But if he says "I want 72, ahem, torpedo tubes dotting the hull", you do start questioning his sanity out of simple professional pride.
Timo Saloniemi