Referendum with higher thresholds are not unheard of. Swiss referendums have a double majority requirement (50% of the popular vote and succeeding in at least half of the provinces). The Montenegro independence referendum had a 55% threshold. The South Sudan had a 51% majority and 60% turnout threshold and I think the East Timor referendum might have had something like that too.

One reason that's not that common is that a supermajority requirement also implies that a minority gets to decide the result and force its choice on the majority. It's a logical consequence and potentially a defensible procedure when you are talking about major, irreversible changes but if you state it like that it does not sit well with democratic sentiment.

Referendum with higher thresholds are not unheard of. Swiss referendums have a double majority requirement (50% of the popular vote and succeeding in at least half of the provinces). The Montenegro independence referendum had a 55% threshold. The South Sudan independence referendum had a 51% majority and 60% turnout threshold and I think the East Timor referendum might have had something like that too.

One reason that's not that common is that a supermajority requirement also implies that a minority gets to decide the result and force its choice on the majority. It's a logical consequence and potentially a defensible procedure when you are talking about major, irreversible changes but if you state it like that it does not sit well with democratic sentiment.