Why wasn't the Bible enough to live by? The answer should be obvious if you consider how most of the colonists (or their ancestors) got to America in the first place: a significant percentage of the original colonists were religious refugees, fleeing persecution from people who believed that the Bible was enough to live by, but had their own opinions of what the rules the Bible gave meant. And when you're trying to build a stable society composed of multiple distinct sub-groups with backgrounds like that, each of whom still have their own distinct ideas about what the Bible means, saying "just look at the Bible" just isn't going to cut it.
And it's not like things got any better in that regard after the establishment of the USA. If we look forward another 40-50 years, we have a record of more of the same, even to the point of direct competition and bitterness between Christians who were nominally brothers in the faith. From the account of Joseph Smith, regarding the time before the experiences that shaped him into a religious leader:
Some time in the second year after our removal to Manchester, [ie.
the late 1810s] there was in the place where we lived an unusual
excitement on the subject of religion. It commenced with the
Methodists, but soon became general among all the sects in that region
of country. Indeed, the whole district of country seemed affected by
it, and great multitudes united themselves to the different religious
parties, which created no small stir and division amongst the people,
some crying, “Lo, here!” and others, “Lo, there!” Some were contending
for the Methodist faith, some for the Presbyterian, and some for the
For, notwithstanding the great love which the converts to these
different faiths expressed at the time of their conversion, and the
great zeal manifested by the respective clergy, who were active in
getting up and promoting this extraordinary scene of religious
feeling, in order to have everybody converted, as they were pleased to
call it, let them join what sect they pleased; yet when the converts
began to file off, some to one party and some to another, it was seen
that the seemingly good feelings of both the priests and the converts
were more pretended than real; for a scene of great confusion and bad
feeling ensued—priest contending against priest, and convert against
convert; so that all their good feelings one for another, if they ever
had any, were entirely lost in a strife of words and a contest about
My mind at times was greatly excited, the cry and tumult were so great
and incessant. The Presbyterians were most decided against the
Baptists and Methodists, and used all the powers of both reason and
sophistry to prove their errors, or, at least, to make the people
think they were in error. On the other hand, the Baptists and
Methodists in their turn were equally zealous in endeavoring to
establish their own tenets and disprove all others.
[I knew] that if any person needed wisdom from God, I did; for how to
act I did not know, and unless I could get more wisdom than I then
had, I would never know; for the teachers of religion of the
different sects understood the same passages of scripture so
differently as to destroy all confidence in settling the question by
an appeal to the Bible.
And this is just talking about a bunch of Protestants. Imagine the confusion if Catholics were thrown into the mix! (And there were and still are plenty of them in America.) Then add in people of non-Christian faiths, and even atheists, and things just get messier...
Having pointed that out, it's still very interesting to look at the Constitution and at early American laws and legal concepts, and see how much of them were drawn from Biblical precepts. A surprising percentage of them have clear roots in Old Testament civil law. But in order to build a strong civilization around them, they still needed to be formally codified as law, originally drafted in the modern, common language, with a clear interpretation.