Charging bears are hard to kill, they have dumped adrenaline and even kill shots aren't immediately effective.
https://www.grandviewoutdoors.com/big-game-hunting/just-stop-charging-bear
Bears are incredibly fast despite their size, so speed of response is essential. Even if a potential victim sees the bear coming, time may be short. In a typical attack, the victim may have as little as .5 to 1.5 seconds to react. Training and practice are essential to survival. But just shooting the bear may not be enough to stop the attack. Bears are tough with heavy muscle and hide and very strong, thick bones. So, the first challenge is for the bullet to penetrate deeply enough to get to vital organs which, when damaged sufficiently, will cause the animal to stop the attack immediately. However, even if the heart or lungs are destroyed, the bear may still have enough oxygen and blood in its system to carry out the attack and kill the victim. Sure, the bear may die, but not before the victim is dead.
The best bet to immediately stop an attacking bear is to turn off its central nervous system, which requires a brain shot or a shot which severs the spinal cord. Since the spinal cord is so small, it makes sense to aim at the brain. But bears have thick, strong skulls shotgun slugs or even rifle bullets may not penetrate. The weakest part of a bear's skull is behind the face, in a small triangular shaped area roughly bounded by the eyes and the tip of the nose. A bullet striking this area has the greatest chance of penetrating the skull, hitting the brain and stopping the attack immediately. But it's a small area, the bear is moving, and the victim is under tremendous stress to respond instinctively before the bear makes contact.