FMJs don’t penetrate straight if there’s anything hard and slanted/sloped in their way, so they can fail even with close-to-decent placement on skulls. They also cause less trauma and bleeding with the same placement (relevant to lung shots, shots to major muscle tissues, and shots near major blood vessels). So, if you’re counting on shock and/or a blood pressure drop to debilitate an attacker, FMJs are going to be worse at inducing that type of stop.
With all due respect to the experience of
@BatteryOaksBilly, I have seen hundreds of gunshot wounds on living folks and in postmortems. There is a huge difference between an FMJ wound and a JHP wound.
Everyone can carry what they want. People can and do carry .22s and put birdshot in home defense guns. But, if you’re preparing for an unlikely and terrible thing to happen (having to put rounds in a person), and you have a gun for which great JHP options exist, why carry something that’s marginal for the task?
FMJ went out of vogue for LE and defensive use in the same way that typewriters, rotary phones, leaded paint, car phones, rolodexes, leaded gasoline, telephone switchboard operators, and the horse-and-buggy went out of vogue. Something much better came along, it became the new standard, and the improvements continue.