Because God is the provider of free will to all of humanity. We don’t possess that ability. By our trying to prevent harm upon another person doesn’t affect anyone’s free will. It may prevent the evil doer from carrying out his evil intentions, but his ability to do evil will not be affected. However, God most likely would prevent evil by forcing a mind change of the evil doer—hence negating free will.
Having said this, I suppose another way God could prevent suffering caused by evil people would be to miraculously intervene in each case. But if He did that in every case, the effect would be the same. The evil doer would be unable to carry out his evil acts—hence the negation of his free will.
In our case, we might prevent a few people from carrying out their evil acts, but that would not result in preventing evil people in general from acting evilly.