When was the idea of providing countries with an option to withdraw from EU first conceived of? Who were the main proponents of the clause and why did they think it to be necessary?
It's mainly because there was no official mechanism to withdraw from the European Union before the Treaty of Lisbon was enter into force.
As mentioned by the author of Article 50, Lord John Kerr (a former UK diplomat), EU leaders then had feared that Austria may withdraw from the EU if the far-right Austrian Freedom Party was elected.
Kerr imagined that the exit procedure might be triggered after an authoritarian leader took power in a member country and the EU responded by suspending that country’s right to vote on EU decisions.
“It seemed to me very likely that a dictatorial regime would then, in high dudgeon, want to storm out. And to have a procedure for storming out seemed to be quite a sensible thing to do — to avoid the legal chaos of going with no agreement,” Kerr said.
Thus, Article 50 was created so that it would provide an official mechanism and rule, such as the 2-year negotiation period, to withdraw from the bloc.
The rules were written at a time when EU leaders feared that Jorg Haider, then leader of the far-right Austrian Freedom Party, could be elected to lead the country.
The regulations were designed to make it easier for the EU to suspend a country that had elected an authoritarian leader whose values were deemed to be at odds with those of the union.