The scribes and the Pharisees were poster children for legalism.
They had lots of rules, but their burdens were not from God. They wanted everything they did to be seen by men. But before we get too self-righteous, while we are under grace, our human nature causes us also to lean toward legalism. We too do things to be noticed by others. Jesus said, if you want to be great, be a servant; be humble.
The people of Israel were so close to seeing the Kingdom of God, but the leaders “shut up the kingdom against men”. Therefore, Jesus took the offer of the kingdom off the table for that generation.
Gentle Jesus is harsh when it comes to hypocrisy: Blind guides and fools!
They were so close to the kingdom, but their blindness caused them to trip over the stumbling-stone. Jesus wanted to gather his people Israel,
“even as a hen gathereth her chickens under her wings, an ye would not!”
Because of their unbelief, Jesus told them he would not return until they say,
“Blessed is he that cometh in the name of the Lord.”
And one day, at the end of the Great Tribulation, Israel as a nation will cry out for Jesus the Messiah, and then he shall return!
As we read the Olivet Discourse in Luke 21, keep in your mind that Jesus is addressing Israel, not the church. This simple principle will keep you from a lot of confusion in your eschatology. More on this later.