Because there are 52 kinds of bait/tackle (and probably more once SB raises the level cap) and a fisherman who wants the ability to aim for particular results needs to keep the vast majority of those on-hand (ideally all of them, if you want maximum flexibility, though you can certainly get away with skipping a few). I'd rather get space for those 52 specific items, than the what, maybe 5 or so (or maybe fewer) generic slots that would be equivalent in size to it.
(After all, bait has no glamour, no color, no materia, not even a NQ/HQ distinction. It's nothing but a stack size. A single number doesn't take up nearly the space of an extensive detailed record of all possible details that any item could ever have. We're currently wasting lots of enormous slots on a very tiny bit of info.)
Everyone gets an extra tab in the currency window that's exclusively for crafting scrips, even though only a minority do high level crafting. And this would be a whole lot more useful. (Not that that isn't useful. It is and I'm certainly glad we have it. But comparatively, it only saves us a few items, as opposed to 52 of them.)
