The most important reason why the authors of the Constitution allowed for amending the Constitution was because they wanted it to be a document that could stand the test of time. In order to accomplish this, they needed to create a way to change the Constitution.
If you look at the amendments (after the Bill of Rights) you will see that they are there largely because of changes in attitudes. If the Constitution could not have been amended, for example, there would have been no way to make slavery unconstitutional when public opinion turned against slavery. There would have been no way to make women have a constitutionally protected right to vote when public opinion came to feel that women should have that right.
Sure, those things could have just been enacted by law, but they would not have been as strong if they were just in statutes. When the Constitution itself is changed, it sends a much stronger message about what we believe as Americans.
If the Constitution could not have been amended, we would be stuck with a document written over 200 years ago and would be unable to change it when needed. This would make our country weaker. So the Framers allowed for amendments because they wanted the Constitution to be able to change when needed. That would keep the country stable since the same basic Constitution could continue to be in force for years, with only a few changes every now and then.