How come conservative parties appear to walk on water?

The stunning and unexpected victory of David Cameron's Conservative Party in the UK, meaning that they will be able to govern alone for the next 5 years, has some similarities to the success of Key's National-led government here in NZ. And hence some lessons for Labour.
Why are UK and NZ centrist voters quietly sticking with the conservative option? It comes down to three things: a desire for national unity and security; perceived economic credibility; distaste for troublesome coalitions.
Middle-ground electors in both countries are feeling, it seems, vulnerable. They are still getting over the 2008 financial crisis, and they are fearful of extremists. They are willing to bear some unpopular austerity in return for the security that conservatives can more believably promise.
In the UK election, this meant that English and Welsh voters swung to the Tories, if only to counter-act the separatists in the Scottish National Party and to head off a Labour–SNP coalition. Meanwhile the Liberal Democrats were hammered severely too, as punishment, I guess, for the compromises and the problems they have undergone as a minor coalition partner with the Conservatives in the 2010–15 government. Voters have now chosen strong majority government.
UK Labour – just like NZ Labour – is thus caught between a desire to compete with the conservatives for the middle-ground issues of economic and national security, and the redistributive demands of the lower-paid. That is, they are torn between their traditional social democracy (blended with contemporary identity politics) and a need to compete on the same turf as the centrist Tories. Like it or not for Labour, the lessons are as follows:
Don't pander (in public) to minority groups demanding separatism, irredentism, or special recognition.
Prove you can work with, and lead, your coalition/support parties and keep them in line.
Show you are not going to spend our way out of problems, especially for the poor (who don't much vote anyway), and that you are committed to fiscal discipline.
Or, if you can't bear to do all of that, and would rather stick with your left-wing ideals, then expect to lose the next election.