It’s a complicated— but, if you listen to scholars like James Tabor, he’d point out that The Gospel of Thomas points to James, the brother of Jesus, as his true successor—not Peter or Paul. This suggests that the Gospel of Thomas may have been tied to the Jerusalem Church, the earliest Jesus movement, which was wiped out after the destruction of the Second Temple in 70 CE. With its fall, Pauline Christianity—which focused on faith in Jesus’ death and resurrection—became dominant, and texts reflecting alternative traditions, like Thomas, lost influence.

Now, Bart Ehrman on the other hand, would likely argue that Thomas wasn’t included in the canon because it teaches a very different theology from mainstream Christianity. Instead of emphasizing Jesus’ death, resurrection, or the coming Kingdom of God, Thomas focuses on hidden wisdom (gnosis)—the idea that salvation comes through understanding Jesus' secret teachings. Since early church leaders were defining Christianity around the resurrection and apostolic authority, Thomas didn’t fit their vision of orthodoxy.

So, between history (the fall of the Jerusalem Church and Paul’s rise) and theology (wisdom teachings vs. resurrection faith), The Gospel of Thomas was left out of the New Testament.

https://jamestabor.com/the-forgotten-brother-of-jesus-part-3