Why did the Framers of the Constitution create a mechanism for amending the Constitution but then make it very difficult to make any changes?

They knew that people were going to change, and that the Constitution had to change with them. In order to be a working document, it had to be changed to reflect what was going on -- they knew, for instance, that slavery was someday going to be abolished, but they knew it was going to be for several generations.

However, they wanted the amendment process to be difficult, so that the Constitution wasn't changed by people's fleeting whims. In other Words, they made the process difficult so that 'popular passions' wouldn't alter the Constitution.