Doubting Thomas,
Because the Lord Himself chose Paul as Juda's replacement. The Lord hasn't changed. He chose those in the OT who would speak on his behalf. He chose the disciples and he chose Paul. He chose before his manifesting in flesh. He chose while in flesh and continues to choose after his resurrection. Jesus told the disciples to wait for the promise of the Father while in Jerusalem. He didn't tell them they would decide who was going to replace Judas.
Because the Lord Himself chose Paul as Juda's replacement. The Lord hasn't changed. He chose those in the OT who would speak on his behalf. He chose the disciples and he chose Paul. He chose before his manifesting in flesh. He chose while in flesh and continues to choose after his resurrection. Jesus told the disciples to wait for the promise of the Father while in Jerusalem. He didn't tell them they would decide who was going to replace Judas.