Unlike most technical pieces of legislation, this is something that will actually affect all legislation, because it will change the composition of the House of Commons. The fact that elections serve as the primary link between society and government makes it exceptional. Given the Supreme Court's recent and further articulation of the idea that there is a constitutional architecture, even though the electoral system is not laid out in the constitutional text, it is of constitutional significance, and although we are not clear from the jurisprudence, that may mean that electoral reform is a change of a constitutional nature.
I happen to think that if a formal amendment is required, as it is under reapportioning seats, it's something that Parliament can do itself. However, that constitutional nature is something else that adds to the exceptional nature of electoral reform.