Why bother passing a law that requires a two-thirds majority to call a new parliamentary election when any future parliament can just pass a law (with a simple majority) negating the requirement and calling a new election whenever it wants?
That was today’s question for Brits, and the answer turns out to be: random politics. Back in 2011 the Conservative Party won an election but didn’t have a majority. They teamed up with the Liberal Democrats to form a government, but the Lib-Dems insisted on the two-thirds law as a condition of supporting the coalition. Why? Because they wanted a full five-year term. The new law prevented the prime minister from calling an election the first time the polls looked good and he thought he might be able to win a majority on his own.
In other words, it was always meant as a short-term solution to an immediate partisan problem, not a permanent change to the constitutional workings of the country. This is why no one cares much that Boris Johnson is bypassing it now with a new law passed by a simple majority.