Why were some gospels left out of the Bible?

They were left out because they are not first-century apostolic testimony - not because an emperor or a council voted the winners in and suppressed the rest. It is true the early Jesus movement was more diverse than a tidy picture suggests, that there were competing texts, and that the group which became orthodox did prevail - but those real facts don't support the suppression story often built on them.

The decisive reason those other gospels were left out was not theology or politics; it was chronology. The four canonical Gospels date to the first century, within living memory of the events. The Gospel of Thomas, the Gospel of Peter, and the Gospel of Judas are second-century or later compositions - the Gospel of Judas, recovered and published in 2006, is generally dated to around the mid-second century, far too late to come from Judas. They borrow the names of apostles who had long since died. The test the early church applied was historical: is this writing genuinely close to the apostles, and has it been read across the churches? The four Gospels passed; the later texts did not.

Worth saying plainly: recognizing which books are early and apostolic is not the same as proving their contents true - that is a further argument, on further grounds. And these Gnostic texts are not worthless curiosities; they tell us much about the movements that produced them. But they are not first-century apostolic testimony, and that, not a suppression campaign, is why they sit outside the canon.