I believe our founding fathers left it that way in the event out own government became tyrannical.
A lot of people like to make that argument, but I don't see it holding water. Why would you need to write in an amendment to assist in the overthrow of a government? Those actions need to be taken by people who are devoted enough to their cause to be willing to suffer the consequences of failure. The United States was established by a revolution that had a very real chance of failure, and that failure would have very likely lead to the execution of many of those who participated. They didn't need a law to assist their efforts, why would anyone else?
And on top of that, the government was intended to be elected, which is itself supposed to be a mechanism for the people to use to throw out bad leaders. If they wanted to initiate the American Experiment of democracy, why would they write in a mechanism to subvert it?