Disclaimer: My own historical 'training' is limited to high school courses in history and historiography, and a layman's interest - mostly though in modern history. What I recall from my courses is a fascinating variety of methods, perspectives and interpretations of even very recent events.
If you read the Wikipedia article on the Historical Jesus, it suggests a very strong consensus in favour of his existence:
The vast majority of scholars who write on the subject accept that Jesus existed
almost universal assent
In recent years, 'no serious scholar has ventured to postulate the non historicity of Jesus' or at any rate very few, and they have not succeeded in disposing of the much stronger, indeed very abundant, evidence to the contrary
Looking at the actual evidence though, it seems underwhelming. I appreciate that even relatively scant evidence may be sufficient to argue for his existence, especially as an ancient historical figure, but I would have thought the available evidence was flexible enough to support a variety of narratives, including those with arguments that he did not exist. Given the importance and controversy of the question, I would have thought there would be lively debate.
So, my questions are;
Is there a consensus as strong as Wikipedia suggests?; and
Why is the 'debate' so one-sided?
Someone is going to point you to the FAQ section on this, so it might as well be me: Did Jesus Exist?.
It include an answer from me, which I might as well plug: here.
To answer your two questions directly:
Yes, the consensus really is that strong. That's the state of the field.
Why is it that strong? Let me venture an answer that touches upon the procedure of how to deal with this question (historical Jesus).
A lot of people, I suspect, come to the question of 'did Jesus exist?' thinking that they should just weigh up the evidence neutrally and decide 'Is it warranted to believe that Jesus existed?' I agree, this seems like a good procedure. But then they feel that the evidence is scarce, not what they wanted, not 'proof' enough.
What if you asked a different initial question: 'Given the data that exists, what is the most plausible explanation for them?'
That, in my view, casts the whole enterprise in a different light. Instead of demanding a certain standard of proof and thinking that such evidences should survive to satisfy our standard, we ask what is the best explanation for the evidences that do survive.
This also 'gets around' what I suspect is a problem for many people, they think that 'Christian texts' are biased and therefore inadmissable evidence. This overlooks the fact that if there were texts about Jesus, they are very likely to be Christian ones, or at least we are very likely to categorise them as 'religious' anyway. Instead, we ask, 'What is a reasonable explanation for the generation of these texts, the contexts they arose in, and the content they contain? What accounts for their origin and usage?'
When you ask those sorts of questions, there are still all sorts of hypotheses that one can arrive at, to provide a narrative of the origin of these texts. Most of those, and the most plausible of those, agree that Jesus was a historical figure. That's why there's a strong consensus.
Is there room for arguments that Jesus didn't exist? Yes, certainly. However, a lot of these, by those called 'Mythicists', are written by those without expertise in the field (not necessarily in itself a problem), and are very poor scholarship, very often bordering on conspiracy-style theories. The combination of these factors puts the case against Jesus' historical existence into a very poor light, further strengthening the consensus. Again, not that cases can't be made against Jesus' existence, but those that are are often very poor.