Why did the framers of the constitution make it so difficult to amend the constitution?

Amendments can be done by one of two proposed ways:
1- a two-thirds vote of both the House and the Senate
2- a national convention called by Congress at the request of two-thirds of the state legislatures

The farmers of the constitution made it so difficult to amend the constitution because they wanted to ensure that the document would endure and last for as many years as possible regardless who was in power.