Why are frozen foods less affected by bacteria?

Freezing in general causes cells to slow down everything. Cells work more slowly at lower temperatures. If you cool down to near-freezing, cells reproduce very slowly. At very lower temperatures the water in the cells will freeze. This can kill the cells or it slows down the oxygenation which degenerates food.

Bacteria are composed of cells. While we freeze it will kill some of the bacteria that can only live at certain temperatures, but it doesn't kill all of it. Freezing keeps food safe by slowing the movement of molecules, causing microbes to enter a dormant stage. Freezing preserves food for extended periods because it prevents the growth of microorganisms that cause both food spoilage and foodborne illness.

However, these microbes can again become active, multiplying under the right conditions to levels that can lead to foodborne illness. Since they will then grow at about the same rate as microorganisms on fresh food, you must handle thawed items as you would any perishable food.

The best way to kill bacteria is to heat it (boiling kills like everything).