Because the speaker has already left the market. A: "Do you think that there are many people at the market right now?" B: "I don't know. When I was at the market earlier today, there were a lot of people there--however, there are usually fewer people there after lunch, so there probably aren't many there now."

Thanks Kevin. You are saying that since my first sentence is in Past Form, so the second sentence must go in Past Form?

The first sentence is actually "You HAD better leave soon." and is used to give advice. It means the equivalent to "It would be better if you left soon." People often leave out the had in this scenario or shorten it to "you'd better" You use "were" because "a lot of people" is plural and that is what the verb is referring to. If the sentence said "There was one person today in that market." you can see that the verb from changes to the singular.