The Pulpit Commentary observes the following:

We can never know why he was absent. He was given to moody fear, and shrank into solitude; and doubtless in many ways and words, as well as those recorded, had implied the wreck of his hopes. Separated from the fellowship of kindred spirits, he augmented his gloom; he was fast tending to unbelief. The state of his mind throughout the Passover week may have been one reason why the apostles delayed their return to Galilee. They may have come frequently to him with their sublime announcement, not once nor twice only.

John 20:24-29 is the climax and final piece in the theological story of John's Gospel. The Gospel begins with the statement that "the Word was God" (anartcular). And completes the Gospel with the doubting Thomas declaring (uncorrected by Jesus), V28, "My Lord and my God." In the Greek, God is "ho theos" (note the articular form) - the first such usage in the John's Gospel.

Now back to the question by asking what might have happened if Thomas had been present at the first meeting of the disciples and Jesus? One assumes that he would have been surprised but as was all else but otherwise unremarkable.

Therefore, I suspect that in the course of divine providence, Thomas was absent to increase the dramatic effect and to provide the crowning convincing proof from the most incredulous person, that Jesus is "my Lord and My God". Without this conspicuous declaration, John's Gospel would be somehow incomplete, theologically.

Ellicott observes for John 20:28,

My Lord and my God.--These words are preceded by "said unto him," and are followed by "because thou hast seen Me, thou hast believed;" and the words "my Lord" can only be referred to Christ. (Comp. John 20:13.) The sentence cannot therefore, without violence to the context, be taken as an exclamation addressed to God, and is to be understood in the natural meaning of a confession by the Apostle that his Lord was also God.

The Pulpit Commentary observes the following:

We can never know why he was absent. He was given to moody fear, and shrank into solitude; and doubtless in many ways and words, as well as those recorded, had implied the wreck of his hopes. Separated from the fellowship of kindred spirits, he augmented his gloom; he was fast tending to unbelief. The state of his mind throughout the Passover week may have been one reason why the apostles delayed their return to Galilee. They may have come frequently to him with their sublime announcement, not once nor twice only.

John 20:24-29 is the climax and final piece in the theological story of John's Gospel. The Gospel begins with the statement that "the Word was God" (anartcular). And completes the Gospel with the doubting Thomas declaring (uncorrected by Jesus), V28, "My Lord and my God." In the Greek, God is "ho theos" (note the articular form) - the first such usage in the John's Gospel.

Now back to the question by asking what might have happened if Thomas had been present at the first meeting of the disciples and Jesus? One assumes that he would have been surprised, as were all, but otherwise unremarkable.

Therefore, I suspect that in the course of divine providence, Thomas was absent to increase the dramatic effect and to provide the crowning convincing proof from the most incredulous person, that Jesus is "my Lord and My God". Without this conspicuous declaration, John's Gospel would be somehow incomplete, theologically.

Ellicott observes for John 20:28,

My Lord and my God.--These words are preceded by "said unto him," and are followed by "because thou hast seen Me, thou hast believed;" and the words "my Lord" can only be referred to Christ. (Comp. John 20:13.) The sentence cannot therefore, without violence to the context, be taken as an exclamation addressed to God, and is to be understood in the natural meaning of a confession by the Apostle that his Lord was also God.