Another reason is that they all say something different - often radically different.
Why that's interesting is that they're all looking at the same evidence. There are essentially three sources for a "life of Jesus". There are the written gospels, including the four in the Bible plus a number of non-canonical versions of the story. There are the references to Jesus and to early Christians in contemporary Roman and Jewish sources. And finally, there is contextual information - documentary and archaeological information about life in Palestine in the first century which can throw light on the written materials.
Two things make the task of using these materials difficult. Firstly, the material itself is fairly sparse. While the Gospels themselves are rich in detail, even the canonical ones are written decades after the events they describe, and they are not consistent. With the other material, the documentary sources are fragmentary and not particularly reliable, while the contextual material requires a lot of interpretation. Hence, what people make of it says as much about themselves as about the material itself.
So here's my plan. I won't review everthing, because you'd all get bored way before I reached the end. Instead, I'll pull out examples of various sorts of Lives. Probably as follows. The first two books provide a kind of historical/theological background, while the rest provide more or less contemporary alternative views.
- The Quest of the Historical Jesus by Albert Schweitzer. Written just on a century ago, this amazing book is a kind of touchstone for everything that came after. Schweitzer reviews 150 years worth of historical and critical scholarship before finally outlining his own conclusion that Jesus is best seen as enacting in his own person the apacalyptic expectations of first century Judaism.
- A New Quest of the Historical Jesus by James M Robinson. Robinson's book, written in the late 1950's, aims to revive research into the "historical Jesus" after it had fallen into neglect in the decades post-Schweitzer. Looks like it worked!
- Jesus the Man by Barbara Thiering. Starting at the most extreme end of oddity, Thiering's book presents us with a kind of "conspiracy theory Jesus" - everything we thought we knew about Jesus is wrong, and she will now reveal the truth. Hers is, at least in approximation, the Jesus of The Da Vinci Code dressed in the garb of scholarship. (At this point, I also give an honourable or perhaps dishonourable mention to Phillip Pullman's bizarre fictional Life of Jesus)
- Honest to Jesus by Robert W Funk. The chief organiser of "The Jesus Seminar" presents us with a thoroughly skeptical view in which Jesus is no more than a clever teacher of philosophy and all the elements of theology attached to him are later additions of the Church.
- Meeting Jesus Again for the First Time by Marcus Borg. Borg is another participant in The Jesus Seminar but he has radically different use for its skeptical conclusions, using them to build a lively and rebellious spirituality.
- Jesus Before Christianity by Albert Nolan describes Jesus through a lense strongly coloured by Liberation Theology, focusing on Jesus social message in both his words and his actions.
- The Challenge of Jesus by NT Wright. Wright is an Evangelical scholar but not afraid to use the findings of scholarship to challenge and revise the popular picture of Jesus, within the overall framework of respect for traditional Christian belief.
- The Jesus I Never Knew by Philip Yancey. This popular evangelical author presents Jesus as if all the preceding books had never been written, drawing solely from the Biblical sources to present a Jesus that any fundamentalist could accept - but who nonetheless challenges the way we live at every point.
That's a long list already and no doubt there'll be some slips along the way. Hope you enjoy it as much as I do.