Why wasn't the bill of rights part of the original consitution?

Wouldn’t these have been pretty simple things? Why would they need to be amended? And ifcthey were amended so soon afterwards, why not just have them in the original consitution?

It’s covered pretty well in that article. The main point is that it wasn’t considered important enough at first by the framers, but it was recognized that without it they wouldn’t get ratification by enough states.

In short,m the Bill of Rights has its own Preamble, which reads in part:

THE Conventions of a number of the States, having at the time of their adopting the Constitution, expressed a desire, in order to prevent misconstruction or abuse of its powers, that further declaratory and restrictive clauses should be added: And as extending the ground of public confidence in the Government, will best ensure the beneficent ends of its institution.

To broaden the scope of the OP’s question a little more, the grade school version of the story of the Constitution tends be along the lines of, “A bunch of really smart guys went to Philadelphia right after we won the Revolutionary War, and produced this immaculate document that unified the country.” That’s a very inspiring story, and it resonates with many Americans, but it’s really wrong on multiple levels.

The Constitution was a product of backroom deals, compromises (some of which are unconscionable today), violent disagreements, and, of course, politics. Today, we often think of the Founding Fathers as a group – like maybe they were a football team or something – but that obscures that they were not all of the same mind on important issues.

To be blunt about it, and use a modern analogy, when the Obamacare bill was debated in the Senate it was found to be short of the necessary votes, so there were backroom deals such as the "Cornhusker kickback." Scrambling to add things in at the last minute thus guaranteed passage of the bill.

The Bill of Rights in a crass sense is sort of like the Cornhusker kickback – “Oh, crap, we’re short of votes. What can we do? Let’s throw in some guarantees of personal liberty to make sure we get this thing passed.” The main difference, of course, is that the Bill of Rights is a heck of a lot more noble than Medicaid expansion. By miles.

The Constitution created a government structure and gave certain powers to that government. It was naively thought that the government would only do what was mentioned in the constitution and nothing else. The bill or rights is a list of things that the government can not do. Many thought this unnecessary because unless it was explicitly mentioned as something the federal government could do then it was understood that they could not do it.

I think that’s really the ultimate reason, and we see how the “if it’s not allowed, then the government won’t do it” line of thinking played out.

My thinking is that a constitution as a document ought to do 2 things- lay out the structure of the government and its operation and guarantee certain rights to any parties to be bound by said constitution.

I also think that the best way to do the second one is to lay explicit prohibitions on what the government cannot do, rather than proclaiming a relatively vague “right” to something. That’s why the US and (unwritten) British constitutions tend to stand the test of time, and why the EU Constitution never really got off the ground. The first two don’t really guarantee anything except that the government will NOT do a handful of things, while the proposed EU constitution tried to guarantee everything to everybody.

This is oversimplified. While some, and perhaps a majority, of the delegates felt that way, many did not. For example, Madison took an expansive view of the Necessary and Proper Clause and in Federalist No. 44 actually explained that the remedy for perceived overreach (under that clause) was the ballot box.