Because that is what Jesus indicated.
This occurred around 70 AD and was great tribulation in and around Jerusalem:
Luke 21:20 “When you see Jerusalem being surrounded by armies, you will know that its desolation is near. 21 Then let those who are in Judea flee to the mountains, let those in the city get out, and let those in the country not enter the city. 22 For this is the time of punishment in fulfillment of all that has been written. 23 How dreadful it will be in those days for pregnant women and nursing mothers! There will be great distress in the land and wrath against this people. 24 They will fall by the sword and will be taken as prisoners to all the nations. Jerusalem will be trampled on by the Gentiles until the times of the Gentiles are fulfilled.
Around the end of the times of the Gentiles is another great tribulation, but this time it's global involving many nations and not just one nation:
Luke 21:25 “There will be signs in the sun, moon and stars. On the earth, nations will be in anguish and perplexity at the roaring and tossing of the sea. 26 People will faint from terror, apprehensive of what is coming on the world, for the heavenly bodies will be shaken. 27 At that time they will see the Son of Man coming in a cloud with power and great glory. 28 When these things begin to take place, stand up and lift up your heads, because your redemption is drawing near.”
Jesus was asked two different questions and they were about two different things. The first one was about things related to the destruction of the temple buildings which He had just previously talked about before they asked their questions. The second one is about things related to His future coming at the end of the age. It is foolish to assume that everything Jesus talked about in the Olivet Discourse is either all fulfilled or all will be fulfilled in the future.
Lk 21
5 As some were talking about
the temple, that it was decorated with beautiful stones and consecrated offerings [of magnificent gifts of gold which were displayed on the walls and hung in the porticoes], He said,
6 “
As for all these things which you see, the time will come when there will not be one stone left on another that will not be torn down.”
7 They asked Him, “Teacher,
when will these things happen? And what will be the sign when these things are about to happen?”
8 He said, “Be careful
and see to it that you are not misled; for many will come in My name [appropriating for themselves the name Messiah which belongs to Me alone], saying, ‘I am
He,’ and, ‘The time is near!’ Do not follow them. 9 When you hear of wars and disturbances [civil unrest, revolts, uprisings], do not panic;
for these things must take place first, but the end will not come immediately.”
THese things above =
The temple being destroyed through war with Jerusalem.
The end of the age didn't come immediately, did it?
1st there is great tribulation regarding the land, Temple and people of Israel and later on the whole world.
20 “But when you see Jerusalem surrounded by [hostile] armies, then understand [with confident assurance] that her complete destruction is near.
21 At that time, those who are in Judea must flee to the mountains, and those who are inside the city (Jerusalem) must get out, and those who are [out] in the country must not enter the city; 22
for these are days of vengeance [of rendering full justice or satisfaction], so that all things which are written will be fulfilled. 23 Woe to those women who are pregnant and to those who are nursing babies in those days! For great trouble and anguish will be on the land, and wrath and retribution on this people [Israel]. 24 And they will fall by the edge of the sword, and will be led captive into all nations; and Jerusalem will be trampled underfoot by the Gentiles until the times of the Gentiles are fulfilled (completed).
34 “But be on guard, so that your hearts are not weighed down
and depressed with the giddiness of debauchery and the nausea of self-indulgence and the worldly worries of life, and then that day [when the Messiah returns] will not come on you suddenly like a trap;
35 for it will come upon all those who live on the face of all the earth. 36 But keep alert at all times [be attentive and ready], praying that you may have the strength
and ability [to be found worthy and] to escape all these things that are going to take place, and to stand in the presence of the Son of Man [at His coming].”