I think you have the two best reasons already listed, but another one could include that John Proctor is guilty of the sin of Pride. He has alientated himself from the church, and made enemies of some townspeople such that he has both ruined his credibility and made himself and his wife an easy target. He doesn't like the Minister and says so openly, was in border disputes with Mr. Putnam, and through his own personality and actions made it so he was less likely to be believed when he did come forward. While this may not have been intentional, as he could not have known the crisis that was coming, when added to the adulterous situation with Abigail, it made it very difficult if not impossible for him to stop the Court from condemning the accused, his wife, and in the end, himself.
Part of the reason that John Proctor is to blame for the events that take place in Salem is that, for a long time, he prioritizes his reputation over his integrity. To admit what Abigail told him -- that the girls are lying -- he would have to disclose the illicit relationship he had with her, and he cares more about keeping his good name, evidently, than clearing anyone else's (until it is his own wife's, and then it is too late). He feels like a fraud -- Miller tells us so when Proctor is first introduced -- but it is important to him that no one else think of him this way. It isn't until the end of the play, when all the damage is done, that he privileges his integrity: he goes to the gallows, unwilling to confess a lie to save his life, and so he finally finds his goodness. However, had he cared less early on about what others might say, how they might view him differently if they found out he wasn't a saint, a great deal of trouble could have been avoided.