In Matthew 24, why did the disciples ask about the end of the world when Jesus spoke of the temple?

I think you are reading it wrong. Jesus did talk about the end of the world. In Matthew 24:13 He says that when you see all these things, earthquakes and so on, the end has not come yet, and in verse 14 that this gospel shall be preached to all the world and then the end shall come, and then verse 15 speaks of the abomination of desolation. Matthew 24 is a little tricky because two themes run through it: the destruction of the Jews in 70 A.D. and the actual end of the world, and they switch back and forth about five times. You can always tell it is the end of the world when the stars fall from heaven; whenever the stars are involved it is the final day of the Lord. When only the sun and the moon are darkened, that is not the final end but the cross, which is also called the day of the Lord. There are two days of the Lord: one at the cross, where God judges Jesus and the Jews, with the sun and moon darkened but the stars still there, and the final one, where He judges the whole world. The same two themes run through Mark 13 and Luke 21, each with its own idiosyncrasies, but that is basically what is going on.