Why did the framers of the constitution create a mechanism for amending the constitution but then make it very difficult to actually make any changes?

They knew that no document could cover every issue, they knew that the nation would change and that needs would change, they knew from history that any concrete document without elasticity would never endure. 

It needed to allow for changes, without making it so easy that it became a mess. If you change too many blocks in a foundation, a building will crumble, right? Same thing with a governments' foundation. It must be changed with great thought, planning, and only when necessary