If you look over the Royal Doors next time you’re in the nave at St. Andrews, you’ll note that there are 12 Apostles surrounding Christ at the Mystical Supper - including Judas! The short answer is because he was present at the Last Supper along with the other 11 Apostles. We know from the Gospels that he had already betrayed Christ at this time, and we see him depicted as dipping into the bowl upon the table. After the bread was blessed and broken, Judas left to fetch a guard to arrest Jesus, when he betrayed Him with a kiss.
Though it may seem odd or even inappropriate to us at times, the icons portray events in our Church that actually happened, featuring people who were really present for the events depicted. If anything, we should look upon Judas and his closeness to Christ in the icon and reflect upon our lives - how close to Christ we might believe ourselves to be while still in error or a spirit of uncharity toward our brethren in the Church and our neighbors beyond.