Just because Lord Jesus was crucified at His 1st coming, INSTEAD OF taking direct REIGN over ALL nations and peoples when He appeared in Jerusalem, shows us that this present world is not the time of His physical Kingdom reign, otherwise His servants would have fought at His 1st coming to prevent His crucifixion. This is actually what He was saying in that John 18:36 verse. Christ's 2nd coming... is what His physical Kingdom reign over all nations will be about.
What came at the cross then? Christ's Kingdom of The SPIRIT.
Luke 17:20-21
20 And when He was demanded of the Pharisees, when the kingdom of God should come, He answered them and said, "The kingdom of God cometh not with observation:
21 Neither shall they say, 'Lo here! or, lo there!' for, behold, the kingdom of God is within you."
KJV
"within you" meaning INSIDE YOU. In your spirit by The SPIRIT. Don't you recall what Jesus said about being "born again" of The SPIRIT, which happens through Faith on His death and resurrection?
In that same Luke 17 Chapter Lord Jesus continues...
Luke 17:22-24
22 And He said unto the disciples, "The days will come, when ye shall desire to see one of the days of the Son of man, and ye shall not see it.
23 And they shall say to you, 'See here; or, see there': go not after them, nor follow them.
24 For as the lightning, that lighteneth out of the one part under heaven, shineth unto the other part under heaven; so shall also the Son of man be in His day."
KJV
Based on what Jesus said in verse 20-21, He continues for that time of His Apostle's day, they will desire to see one day of His Kingdom back then, i.e., "one of the days of the Son of man," but they won't see it, because...
As lightening shines from one part of the sky to the other, so shall the day of Christ's future SECOND COMING be. And by that Jesus was pointing directly to His future RETURN to this earth, His 2nd coming, which is when He will take LITERAL PHYSICAL REIGN over all nations on earth.
What then happened when The Gospel began to be accepted by nations in the West, the early Church began thinking what you are saying, that since Lord Jesus died on the cross and brought His Kingdom of The Spirit, that meant NOW Christ's people were to take rule over the nations, and thus the whole world. Certain political Christian groups today still... think this for this present world time, and are actually helping Satan's host to build a New World Order.
And the fact of the Revelation beast kingdom prophecy that is coming in our days with that New World Order, that is major evidence that Christ's physical Kingdom is not yet, and definitely not of this present world time. The "dragon" is coming to reign over all nations for 42 months, as written there in Revelation 13. Have you missed that?