Because actually they did not know what is right and what is wrong, what is from god and what is from men. Because they afriad god and either man.

They were confusing to Jesus' question, the question quoted their mind with no answer.

Because their mind is for man and nor from god. They though that they were clever then they were not.

Final because they understood the authority of Jesus got "From God".

to as human thinking, Zaccheaus perhalf knew and got the news about Jesus, like famous and great person, and yes that was why he was joyful.

To as christian view, you can see that Zaccheaus was seeking Jesus Christ not just to see or have a look, but what Zaccheaus did was he was really love to see until he sacrified his body to claim the tree and give all his heart to only how he can see Jesus enough. It seems Zaccheaus forgot every thing focused on Jesus only one. 

Zaccheaus perhalfs though that to just see Jesus, Lord Of man, Mesiah and the saveior, he is blessed.

Finally, because Zaccheaus knew that the lord is come to his house neither be blessed like he was.