Theophylact of Ohrid

Now those things which began from the Word, end in the Word. For by this he implies that up to a certain time the Law and the Prophets appear, as here Moses and Elias; but afterwards, at their departure, Jesus is alone. For now abideth the Gospel, legal things having passed away.

Catena Aurea by Aquinas ↗

John Damascene

This also our Lord commands, since He knew His disciples to be imperfect, seeing that they had not yet received the full measure of the Spirit, lest the hearts of others who had not seen should be prostrated by sorrow, and lest the traitor should be stirred up to a frantic hatred.

Catena Aurea by Aquinas ↗

Ambrose of Milan

They then departed, when our Lord's manifestation had begun. There are three seen at the beginning, one at the end; for faith being made perfect, they are one. Therefore are they also received into the body of Christ, because we also shall be one in Christ Jesus; or perhaps, because the Law and the Prophets came out from the Word.

Catena Aurea by Aquinas ↗

Bede

And while the voice was being spoken, Jesus was found alone. When the Son began to be designated, the servants immediately departed, lest the paternal voice be thought to be addressed to them. Alternatively: as the voice was made over the Son, He was found alone, because when He reveals Himself to the chosen, God will be all in all (1 Cor. 15), indeed He with His own, Christ through all will be one, that is, the head will shine with the body. …

On the Gospel of Luke ↗

Bonaventure

And as the voice came, etc. Here lastly, after the consummation of the contemplation, the Evangelist adds its withdrawal, concerning which the Evangelist introduces two things: the first is the return of the accustomed vision, the second is the concealment of the vision that was shown. First therefore, as regards the return of the accustomed vision, it is said: And when the voice was made, Jesus was found alone, namely without Moses and Elijah, just as He was before with them. …

Commentary on Luke, Chapter 9 ↗