Why is the "Jesus Myth" theory universally disregarded?

You are making a lot of assumptions here. Some scholars do believe that Mark was a disciple of Peter, but this doesn’t mean that Mark relied solely on Peter.

From here:

Actually we get that from Eusebius, who is purporting to quote Papius. I’m not entirely sure how we can have confidence in his opinion of Mark and how it came about. Papius was born somewhere around 70 AD, which is when scholars date Mark to. He is supposed to have written around 100-130 AD, which is far too late to really give any credence to his knowing Mark, really.

Further, as Ehrman loves to point out (in Misquoting Jesus), the early Christians had a habit of altering texts - many of the early Church figures try to dissuade the copyists from adding things to their text. The most famous one that I can think of is the end of the Revelation.

Eusebius didn’t think that Papias was very bright, apparently because he passed on any fantastic story that he heard. Little of Papias survives, but there is one story about how Judas became so incredibly fat and disgusting that he could not get through a space where a chariot could pass.

I just read this entire thread, and found it fascinating. Much of the core argument against Jesus mythicism is that there isn’t a coherent hypothesis of how Jesus was historicized into a man.

I felt the need to repeat two crucial posts by VinnyJH. The idea he presents is worth another look by all rational observers. At the very least, it goes to show that the Jesus myth theory is not simply inane. It raises some profound and important questions that noone can readily dismiss.

I would not dismiss his conjecture out of hand.

I would note that Paul spends a lot of effort condemning the actions of people who are actively preaching different things about the same character. This strongly implies that there was a tradition of a man, Jesus, that preceded Paul’s preaching. Otherwise, you have a guy inventing a religion that begins to encounter separate traditions across widely separated locales in just a few years without the benefit of modern media or even a printing press. It also ignores Paul’s own statements regarding his feuds with the church leaders in Jerusalem. A claim of a complete lack of historicity requires that Paul was able to get a religion founded that grew large enough to spin off doctrinal divisions, but that permitted the believers of those disparate beliefs to reconcile with each other in a very short time.
Impossible? Perhaps not. Implausible? Very.

You might be correct that the diversity of the traditions proves that Paul couldn’t have been the founder of the movement, but I don’t see that it makes it any more likely that the competing traditions were traditions of a man rather than traditions of a heavenly being who was known through revelation and appearances. On the other hand, if you look at the history of the Mormon church, there were disputes and factions from a fairly early point so I’m not sure how much can be inferred from Paul’s feuds.

Are there references to Joseph Smith getting into fights with other people preaching about the Book of Mormon when he arrived in Ohio or Illinois or Missouri? Paul’s feuds say nothing about a historical Jesus. They do point to a larger movement that existed separate from Paul, calling into question those theories that claim that Paul “invented” Christianity.

Paul tells us that others saw the risen Christ before him. On the other hand, he also says that his message came directly from God by revelation and that no man added anything to it. I think that makes it very hard to determine how much of what we now think of as Christianity existed before Paul came along and how much of it was due to his theological creativity and imagination, particularly since Paul is our earliest source and he tells us next to nothing about what the movement looked like before he came along.

Paul says that after his conversion, he went out and preached for three years before he bothered to go up to Jerusalem and he didn’t go there again for another fourteen years, at which point Peter, James and John were still there. Most of Paul’s letters are to communities that he founded. I agree that there was some larger movement, but I’m not sure that it can be established that it was very much larger. I’m not sure that Paul “founded” or “invented” Christianity, but he may nonetheless have been responsible for much or most of what we now understand to be Christianity.