>But what exactly is wrong with letting the voters decide on amendments??

In theory and on paper? Nothing.

The issue is with how it's implemented and the threshold for passing. The US Constitution takes 2/3 of the House and Senate, and then 3/4 of the states to agree to it before an amendment goes through. In PA all it takes is 50% + 1 at every step of the process. The state GOP had been abusing the process in order to pass unpopular legislation that they wouldn't be able to muster enough votes to override a D governor's veto. For something as serious as amending the construction, a larger consensus should be made, not a skin of the teeth majority.