And if those days had not been cut short, no flesh would be saved. But for the sake of the elect those days will be cut short. -Matthew 24:22
It would have been impossible for man to destroy all flesh on the planet until at least around the late 1950's.

Thus declares the Lord... ...I am about to make Jerusalem a cup of staggering to all the surrounding peoples. The siege of Jerusalem will also be against Judah. On that day I will make Jerusalem a heavy stone for all the peoples. All who lift it will surely hurt themselves. And all the nations of the earth will gather against it. -Zechariah 12
Israel, the nation of the Jews, did not exist until 1948, So this could not have been fulfilled before that.
Nothing, since World War II, has changed the world more or made it more dangerous than the establishment of the Jewish nation in the Middle East.

And this gospel of the kingdom will be proclaimed throughout the whole world as a testimony to all nations, and then the end will come -Matthew 24:14
Again at no point in history has the technology existed until at least the 1950's for the gospel to be spread to the entire world. With the invention of Satalite TV The Internet and Shortwave Radio it can reach places far easier than ever before.

And I will grant authority to my two witnesses, and they will prophesy for 1,260 days, clothed in sackcloth. And if anyone would harm them, fire pours from their mouth and consumes their foes. They have the power to shut the sky, that no rain may fall during the days of their prophesying, and they have power over the waters to turn them into blood and to strike the earth with every kind of plague, as often as they desire. And when they have finished their testimony, the beast that rises from the bottomless pit will make war on them and conquer them and kill them, and their dead bodies will lie in the street of the great city that symbolically is called Sodom and Egypt, where their Lord was crucified. For three and a half days the peoples and tribes and languages and nations will gaze at their dead bodies and refuse to let them be placed in a tomb, and those who dwell on the earth will rejoice over them and make merry and exchange presents, because these two prophets had been a torment to those who dwell on the earth. But after the three and a half days a breath of life from God entered them, and they stood up on their feet, and great fear fell on those who saw them. -Revelation 11
Until the invention of Satalite TV it would have been impossible for people of all nations and tribes and languages to actually see the two witnesses.

In the Last Days, Knowledge Shall Increase.(Daniel 12)
Never in the known history of the world has knowledge increased at such a rate as in the last 100 years.

For then will I turn to the people a pure language, that they may all call upon the name of the LORD, to serve him with one consent. -Zephaniah 3:9
Before the restoration of Israel in 1948, Hebrew was a dead language. Hebrew is Now spoken throughout Israel.