Why if God destroyed the earth with a flood does He not destroy evil civilizations in the past or even now with another thing?

God did destroy evil civilizations after the flood. God has used a gamut of different tools to destroy those civilizations that were against Him. The flood encounter begins in Genesis 6:13 when God makes a covenant with Noah and promises to destroy the rest of mankind from the earth because of their evil ways. One of the next known mass destructions from God upon a nation of people is found in Gen. 18:20. This is the well known account of Sodom and Gomorrah. “Then the Lord rained on Sodom and Gomorrah brimstone and fire from the Lord out of heaven, and He overthrew those cities, and all the valley, and all the inhabitants of the cities, and what grew on the ground,” (Gen. 19:24-25). Let’s not forget what our Lord did to the Egyptian people when the king would not release the Israelites from bondage beginning in Exodus 7. God has not always used supernatural events to destroy people groups. All throughout the book of Joshua God uses the Israelite people to destroy people groups as they claimed their promised land (See Joshua 6 on).
So why doesn’t God destroy evil civilizations in our age? That question cannot be answered by anyone because God has not revealed the answer. We do know that we are in the last days before the return of Christ and that we are in a different age then those who lived during the Old Testament times described above. We are living in a new part of God’s plan. While we may desire for God to rain down fire and brimstone on our enemies we must remember that we are also sinners who deserve the same death as our enemies, but God by His mercy has given us the gift of life in Jesus Christ and thus we are saved from the wrath from Him that we deserve.