Why it taking so long to reach God's Kingdom on earth?

I don't think it has stalled at all.

Bible prophecy (and I won't get into it here, its too complicated, but email me if you want details) is specific that nothing regarding God's kingdom was even supposed to start or have any beginnings until the 20th century.

There is now only one religion that has totally conquered racial, ethnic, and nationalistic warfare.

Where ever you go on the earth, members of this religion have overcome and avoided things like participation in the World Wars and the ethnic and racial strife of places like Bosnia and Rwanda. If you go to the middle east you will find Arab and Jewish members of this religion worshiping and living together peacefully.

As part of the many things that Jesus said about God's Kingdom, he identified that his true disciples will have love among themselves. (John 13:35) From a few thousand in the early 20th century there are now over 8 million of them showing love as described above and another 8 million possibly in the wings.

God's Kingdom on earth is growing.

Actually Eternity is constantly and instantly in the present. It is the human mind because of time that is to slow to comprehend. Eternity can be an instant revelation. Life is already Eternal for Life is constantly and instantly in the present. We either decide to enter or not. The human mind is fragmented because of time and therefore must have the Single Educator to know Eternity. God will never force Himself or Eternity on anyone so those who do not decide to enter will cease to exist even in memory after judgement as if they never existed at all. Those who do not enter cleave to illusion and take on its nature which is to pass away. Illusion is whatever appears and disappears like a mirage so time can measure it while THE TRUTH (Reality) is instantly in the present. God is the Imageless Spririt of Life. Life is awareness and awareness is Spirit.

Fact is, mankind still seeks power, greed, and control. Some are ready to move forward into a more spiritual way of life, but some are not and are selfishly still mired in materialism as their only reality. Anyways, light your candle to dispel the darkness around you and be an example to the world of what you want to see.

In the first century, there were some who apparently felt that God should have fulfilled his promises sooner. They had become so impatient that they even considered God to be slow or late. On this account, the apostle Peter had to remind them that God’s view of time is quite different from ours. Peter writes: “Let this one fact not be escaping your notice, beloved ones, that one day is with Jehovah as a thousand years and a thousand years as one day.”—2 Peter 3:8.

According to this way of figuring time, an 80-year-old person has lived only about two hours, and all of mankind’s history has lasted only about six days. When we look at things from this perspective, it becomes easier for us to understand God’s way of dealing with us.

God, however, is not indifferent to time. On the contrary, he is very time conscious. (Acts 1:7) Peter, therefore, goes on to say: “Jehovah is not slow respecting his promise, as some people consider slowness, but he is patient with you because he does not desire any to be destroyed but desires all to attain to repentance.” (2 Peter 3:9) Unlike humans, God does not feel pressured to do things as if time were running out for him. As the “King of eternity,” he has a marvelous overview and can determine when in the stream of time his actions will do the most good for all involved.—1 Timothy 1:17.

Sin and unbellief are the reasons. Those things won't be corrected until the 1,000yr

reign of Christ, after the tribulation.