Because you cannot read the heart of someone else. Only God can read hearts. He alone knows how that person was raised, what truths, if any, he was taught and how he applied those in his life.
If anyone were entitled to seek revenge, would it not be Jesus Christ, who was unjustly accused and crucified for upholding the truth? We follow His example.
“I am still wondering why I can’t get revenge and only God can get vengeance?”
A little further....God seeks that 100th sheep that left Him...not satisfied with the delightful 99 that stay, as precious as they fully are. If you seek vengence, that 100th dies before he can repent. The 100th is lost, requires a greater level of mercy, not having the longest possible chance to come around of his own free will, as designed, to repent and give Glory to God. And YOU - you need the opportunity to let go and let God, and forgive - for your own true Joy as God designed you to experience. But, you might get to feel regret and repentence still, afterward, if given enough time, just not Joy soeasily due to the added baggage of vengence.