Why did the writers of the constitution allow for amendments?

When they wrote the Constitution, they knew they were writing it for a bunch of people they wouldn't know - posterity. The Framers were smart enough to know that times were going to change and so were beliefs and laws. They knew that someday the evils of slavery were going to be recognized, women were going to be considered social equals, and the like. So they included the amendment process so that the Constitution could change with the times and adapt to new beliefs, or, in other Words, withstand the test of time.