Naked self-interest rules, pure and simple.
Well may the Government try to blame a lack of consensus amongst Parliament's component parties as reason for not implementing the recommendations of the Electoral Commission-conducted review of MMP.
That rationale is just a little too convenient, however. When it comes to consensus, National is the one which refused to budge in its opposition to arguably the commission's most important and most controversial finding - that the anomalous, outdated one-seat threshold under which minor party list candidates can coat-tail into Parliament on the back of a MP winning an electorate seat should be abolished.
National has refused to budge because retention of the one-seat threshold gives it greater chance of getting enough backing by way of minor party seats sufficient to give it a majority in the next Parliament.
Such a stance is totally indefensible. But it is also completely understandable.