I think body armor can only do so much against certain kinds of weapons (such as disruptors at their highest setting--which can probably vaporize both you and
your armor). It could also be that there are limits to personal forcefields (they may interfere with transporters perhaps). In such instances, it could be that the technology is probably there, but hasn't been perfected yet for general usage. Maybe in the 25th-Century...
But that isn't to say that Starfleet security/combat personnel shouldn't
be wearing some sort of protective gear, though (like they did in TMP). If nothing else, Starfleet crews still frequently encounter civilizations using bladed weapons or even old-fashioned "slugthrowers" at times, and body armor would be definitely handy in those instances, IMO.