The problem with body armor is that it exacts a toll on the user. The more effective the armor, the harder it is for the wearer to adapt. Kevlar body armor weighs a lot and is far from breathable, so spending 8 hours wearing a 20lb vest can get quite tiring. Body armor capable of stopping rifle fire is oppressively heavy to wear, and a careful shooter can still exact a lethal injury by aiming for the head , groin,or armpit.
In the Trek verse of things, the situation is compounded because of the power requirements of an energy field. Making a nifty Stargate-type Goa'ould personal energy field means building a power source small enough to be portable, which would be tantamount to running around with a small power plant attached to your belt. I'd hate to be that guy if the power cell is damaged in battle -or if it overloads and goes boom