Of course, budget is also an issue. Why spend money on special uniforms and armour when a guy armed with a phaser rifle conveys the message to the audience at a fraction of the cost? The fact that someone was willing to spill some money to make those outfits for the guys in Nor the Battle to the Strong was surprising. Less so with the MACOs, since they were covered in the new season's budget.
Security officers in the TOS movies did wear armour, but only when they were on shipboard duty. No one that took part in the assault on Nimbus III in Trek V was wearing the armour. In fact, in Flashback when Sulu calls security to the bridge, those guards are only wearing the enlisted uniform.