Jesus arrives in Jerusalem for the Passover, but he has come early, the first day of the week, and Passover won't begin until Thursday or Friday evening. As it gets closer to Passover, the crowds of Jewish people coming to celebrate will grow. And the danger that Jesus could cause a major uproar or rebellion by publicly claiming to be the Messiah will grow, also. So if one wants to get rid of Jesus, the sooner the better.
But there is a problem. Just as with modern cities, where the closer to town vacationers want to stay the more expensive the lodgings, so was it also the case in Jerusalem. So most visitors would stay in the outlying towns at night and come to the city during the day. Jesus and his disciples, along with the crowd of Galilean Jews, did likewise, with many of them staying in Bethany, about two miles away.
Thus, from morning to evening Jesus will be surrounded by a large group of Galilean Jews with whom he is very popular. And the more he teaches and heals people, the more his popularity will grow with Judean Jews and even native Jerusalemites. To try to arrest him during the day is just asking for trouble. And at night, he is staying two miles from the city, much too risky for the Temple guards to pull off a secret night arrest successfully.
Another opiton would just be to inform the Romans that Jesus is a dangerous insurgent who must be done away with. But this might bring the same result - a large, bloody riot.
At this point the ruling class in Jerusalem, the priests, have to hope that somehow they will find a way to isolate Jesus from the crowd.