It's my opinion, based on what I've read, that the Gospel of Thomas isn't in the Bible because 1) it's older than any of the other gospels and 2) Jesus presents himself ONLY as a great teacher. There are no "teaching stories" (ala storylines/timelines not parables per se) in the GoT, it is a collection of sayings of a wise master, such as many people collected in those days.
When the Council of Nicea (sp?) came together to create the Bible as we know it, they wanted non-controversial books that presented their POV - that Jesus was god, god was a male diety etc. The GoT does not say anywhere within itself that Jesus presumed to identify with divinity. Therefore, the GoT did not fit their needs (to consolidate a firm set of stories about Jesus) and it became grouped together with the other rejected writings and stories and collections about Jesus, commonly called the apocrypha (sp?).
It's a funny question, because so much of what was written about Jesus was excluded, even if it was decades older than canonical biblical material, because it didn't present the POV of the Council.
The victors write history, but who's to say that the history that was written is actually what happened????