He was unprepared.

And when the king came in to see the guests, he saw there a man which had not on a wedding garment [Matt. 22:11].

"What is that wedding garment? The King’s invitation is for everyone, but there is a danger of coming without meeting the demands of the King. That wedding garment is the righteousness of Christ which is absolutely essential for salvation, and it is supplied to all who believe. The apostle Paul speaks of this imputed righteousness: “But now the righteousness of God without the law [that is, apart from the law] is manifested, being witnessed by the law and the prophets; Even the righteousness of [from] God which is by faith of Jesus Christ unto all and upon all [it comes down upon all] them that believe: for there is no difference” (Rom. 3:21–22). All have to have a wedding garment."

And he saith unto him, Friend, how camest thou in hither not having a wedding garment? And he was speechless [Matt. 22:12].

"Notice that he was speechless! I hear some folk say that they don’t need to receive Christ, that they will take their chances before God, that they intend to argue their case. Well, our Lord said that this fellow without the wedding garment was speechless." McGee, J. V. (1991). Thru the Bible commentary: The Gospels (Matthew 14-28) (electronic ed., Vol. 35, p. 106). Nashville: Thomas Nelson.

"22:11 a wedding garment. All without exception were invited to the banquet, so this man is not to be viewed as a common party-crasher. In fact, all the guests were rounded up hastily from “the highways” and therefore none could be expected to come with proper attire. That means the wedding garments were supplied by the king himself. So this man’s lack of a proper garment indicates he had purposely rejected the king’s own gracious provision. His affront to the king was actually a greater insult than those who refused to come at all, because he committed his impertinence in the very presence of the king. The imagery seems to represent those who identify with the kingdom externally, profess to be Christians, belong to the church in a visible sense—yet spurn the garment of righteousness Christ offers (cf. Is. 61:10) by seeking to establish a righteousness of their own (cf. Rom. 10:3; Phil. 3:8, 9). Ashamed to admit their own spiritual poverty (see note on 5:3), they refuse the better garment the King graciously offers—and thus they are guilty of a horrible sin against His goodness.

22:12 And he was speechless. I.e., he had no excuse." MacArthur, J., Jr. (Ed.). (1997). The MacArthur Study Bible (electronic ed., p. 1434). Nashville, TN: Word Pub.

As a reminder Cross Reference is number 7 on Satan, Inc. (TOL Heretics list) in the Misc. category.

Jerry Shugart ...[I]t was he who did not overcome the world who was not wearing the wedding garment:

"For whatsoever is born of God overcometh the world: and this is the victory that overcometh the world, even our faith. Who is he that overcometh the world, but he that believeth that Jesus is the Son of God?" (1 Jn.5:4-5).

Right, we overcomers. We have overcome: the world (John 16:33), the flesh (2 Pet. 2:19, 20) and the devil (1 John 2:13, 14).

Cross Reference That is merely saying you can't be tempted.

Where did you get that idea? 1 Co 10:12

Why was the friend not wearing his wedding garment?