Because it was the last motion proposed at the Constitutional Convention of 1787, and the delegates were mostly satisfied with what they'd drafted; most of them also thought it was unnecessary, since the states protected individual rights, and there was no reason to specify a list of freedoms the federal government could not violate when it was not authorized to take any away.
Now, the Convention was called in the first place because the Articles of Confederation, which had been the basis of the federal government's operation since 1781, was proving quite inadequate for governance. States were not required to provide tax revenue, which in turn had no effective mechanism for management or expenditure. Four years after the end of the Revolutionary War, they were still paying it off. Regular bills could only be passed with nine out of thirteen states consenting, and amendments had to be unanimous.
So in 1786, Virginia called for a convention addressing interstate conflicts, which itself passed a resolution calling for a convention to improve the Articles of Confederation. Once that convention began, however, it was clear that most of the delegates regarded the Articles of Confederation as fatally flawed, and their goal was to create a new document that would fit their needs better.
The convention devoted itself to working out several key points of the structure of the federal government, and its relationship with the states; the idea that the legislature would be separate from the executive and judiciary was already accepted, as was having two houses in that legislature -- one representing the common people, and the other being a more elite, thoughtful chamber. As they settled the details, the delegates got more irritated and weary (you try sitting in a closed room with no AC in Philadelphia in August), and wanted to send the final draft out. That last motion mentioned above was about civil trials; George Mason proposed a bill of rights dealing with the rights of the people, but the only other vote for that was Elbridge Gerry's. The draft was approved and sent to the states; then the fight for ratification began, but this is not the time to cover that in detail.
The Constitution was ratified by the requisite nine states in September 1788, many with the understanding that a series of amendments protecting the rights of the people would be high on Congress' priority list, and more followed thereafter. Many opponents of the bill of rights began to support it, recognizing that many people wanted that explicit protection, and desperately wishing to avoid another constitutional convention setting them at square one. James Madison was one such opponent, and he submitted twenty proposed amendments to Congress; that was condensed and cut into twelve amendments by the House and Senate committees.
The first two were never approved; the other ten are what we have today. Article the First, as it is known by historians, dealt with Congressional apportionment; Article the Second prohibited Congressional pay raises from taking effect until after the next election, and was initially unable to garner even majority support, with six states ratifying it; Ohio joined that list in 1873, and Wyoming in 1978. Over the next 14 years, a movement spearheaded by Gregory Watson managed to push it through thirty more state legislatures, and in May 1992 it became the Twenty-Seventh Amendment. Unfortunately for its proponents, Congress is just as able to increase its cost of living adjustments, and while Watson and Congressman Bob Schaffer of Colorado challenged those automatic increases, the Supreme Court ruled that Schaffer had no standing, as he could not demonstrate any harm from a higher COLA.