Peter told them why someone must replace Judas.

Acts 1:15-17

15 During those days Peter stood up among his fellow believers. There were at that place a group of about 120 of Jesus’ followers. Peter said, 16 “My fellow believers, there are words that King David wrote [MTY] in the Scriptures long ago that needed to be fulfilled {to happen as he said they would}. The Holy Spirit, who knew that Judas would be the one who would fulfill those words, told David what to write. 17 Although Judas had been chosen {Jesus had chosen Judas}, along with the rest of us (exc) to serve as an apostle, Judas was the person who led to Jesus the people who seized him.”