Why does lichess ban players? Few days ago I was watching a game review on lichess and found out one of the player's account was banned. Any idea why lichess does that?
First of all, lichess rarely bans players. What they usually do is flag their profile (usually for using chess computer assistance), remove their rating, and prevent them from p laying in tournaments and/or rating games. This is not a full ban.

To answer your question:

Lichess flags players for a variety of reasons, but (as mentioned above) they usually flag players for using chess computer assistance. They may also be flagged for sandbagging - artificially increasing or decreasing their rating.

If you want to know why are particular player was flagged, check their profile. There should be a large red banner informing you what has happened.

If you are wondering how the process of reporting/flagging players works, that would be a separate question.