Perhaps you should think of it as difficulty, instead of reluctance.

First, the Constitution was a very big step from the Articles of Confederation . The 1787 Convention in Philadelphia met to try to correct what everyone saw were problems with the loose federation of the states. Many things were not working well, like international relations, diplomacy, trade, collection of taxes to pay off the debt from the Revolutionary War. But instead of small changes, which might have left a system vaguely like the present European Union, the Constitution created a federal government with much larger powers , in effect creating a country. In doing so, the framers had to pacify the fears of the states- especially the smaller states- that the federal government would aggrandize its power at their expense. So, there were safeguards put into it to make this difficult. There was the tri-partite structure, with the executive, legislative and judicial branches acting as checks on each other, There was the Bill of Rights- that 2nd Amendment language about "well-regulated militias" was aimed at the states' fear of a Federal army.

There was also the general fear of what had previously been a problem with a democracy- that a majority faction could take control and tyrannize the minority. Of course, if that faction could easily re-write the constitution, it could keep itself in power forever (and this has actually been a problem- a number of presidents of supposed democracies in the past hundred years have altered their governments to be able to become presidents for life) So, the framers made it quite difficult to amend the constitution.

As you likely know, there are two ways to change the Constitution: ratification of amendments by state legislatures, and by a constitutional convention. The state-by-state ratification of amendments is what has actually happened, and sometimes relatively quickly: the 18th amendment passed pretty easily in 1919, and was pretty easily also repealed in 1933. When there have been fierce political divisions of course that has not been possible: the 13th Amendment,, ending slavery, would not have been possible in 1850.

There has never been another constitutional convention, and that is unlikely: that convention would not be limited to settling only specific questions, but could decide its own agenda. It might seem like a good idea to, say, call a convention to quickly enact the Equal Rights Amendment, but that convention might also decide to give the federal government the right to issue driver's licenses, appoint police chiefs, or allow a president-for-life. Considering the huge changes the 1787 Convention made in the government, fears of another convention are valid.