I think it was Constantine who decided that the religion should be more "focused"...because there were so many different Christian groups, each with their own sacred texts, and the beliefs all differed.
So he called that first meeting in the 4th century...the council...and that's when they decided they needed to put together a book and decide what the official beliefs were going to be, and therefore, which books they should include.
(Why Constantine wanted this is a whole other digression, for another thread)
Also, it was a way to differentiate this religion from Judaism.
For those first few centuries, many considered themselves Jewish people who followed a new Jewish leader. But that didn't go so well with the Jewish people I think, so at some point, they had to figure out that this was a new, different religion with a new name and a new set of beliefs--and if so, they needed their own texts to show that and couldn't rely on oral tradition anymore.
Why had they not put a book together earlier?
Well...the different groups were, indeed, following their own "books"--scared scripture from different disciples...the Ebonites, the Marcionites, the Gnostics, and also, the group that was close to what we consider Orthodox today. But no need to compile them all in one book until Constantine wanted the religion more unified.
He also wanted to promote the religion, so he new it needed a book to help it spread. Good PR and all :-)
Also...I think for many, they thought in the first 1-200 years since Jesus died, they assumed the world was going to end soon anyway--indeed, many thought it would happen soon after Jesus died because he said so (something about "it will happen to some of you in this lifetime" etc)...so they didn't have a need to write anything down and collect it into a book because they figured it was all going to be over any day now...
But after a few hundred years they could then see that the end wasn't as imminent as they had thought...and might be a long, long time away...so better get the religion organized.
Those are my thoughts...from what I've read and from what seems logical.