Although many amendments have been proposed, only twenty-seven amendments have been added to the Constitution. Why have there been so few amendments?
While the United States does not have a provision for entrenched sections of its Constitution (in other words, sections which are not subject to amendment), the structural impediments to any routine amendment are extremely high and difficult to overcome. Amendments require action by two-thirds votes in both of the chambers of Congress, followed by ratification conventions in a super-majority of states. This makes the process of amendment complicated and has limited the number of amendments successfully passed.
A second reason for the small number of successful amendments is the inherent flexibility of the US Constitution. Only a few pages long, the Constitution provides only the most basic structure to government, leaving most issues to be resolved by statute law. In his important study of US constitutional amendments, The Proposed Amendments to the Constitution of the United States During the First Century of Its History, Herman Ames opined that more amendments had not passed because the basic ideas they were proposing ultimately "found a place in that unwritten constitution that has grown up side by side with the written document."
The Founding Fathers wished the Constitution to be a living document; that is, they wanted future generations of Americans to be able to change it as necessary. The Founding Fathers, for instance, could not possibly have imagined something so complex as the Internet we use today, but they knew that future Americans needed the tools to modify their government to deal with issues as they arose.
Even so, they also realized the danger in modifying the Constitution too frequently or too easily. There are many cases in history of dictators seizing power through totally legal means, exploiting flaws in the structure of government in order to become far more powerful than they are supposed to be. As such, modifying the Constitution with an amendment is possible, but it is meant to be very difficult, so that only a wise and just law (such as outlawing slavery, or giving women the right to vote) can be passed by will of the people.
Finally, the US government functions by giving states enough power to be able to make up their own minds on many topics without requiring federal government approval. This is why gambling is legal in Nevada but not in Utah, for example. The Constitution is meant to give power both to the national government and to state governments so that there can be a balance of power throughout all levels.
There are two main reasons for this.
First, the Framers of the Constitution made it very hard to get amendments passed. An amendment must (in the most common procedure for amending the Constitution) be approved by two-thirds of each house of Congress and then ratified by three-fourths of the states. It is extremely difficult to get that kind of a supermajority for anything.
Second, the Constitution is meant to be a very basic document. It lays out the bare bones of American government but very rarely gets into the creation of specific policies. The one time the Constitution was changed to make a very specific policy was with Prohibition. For this reason, there is a general reluctance to use the amendment process to institute policies that ought (in the minds of most Americans) to be made through Congressional statutes.
The process by which the Constitution is amended is very difficult, time consuming, and overall tedious. This is to prevent random amendments from being applied, either giving the government or the people more power than is thought to be necessary.
If the government had the ability to add or take away from the constitution easily, then in theory a President could easily becaome a more powerful ruler.
If the people had the ability to add or take away from the consitution easily, then in theory they would be able to take over the government offices, and create chaos.
Either way, other amendments have not been deemed necessary enough to be made apart of the constitution.