Because you can't get there from here.

FPP politics, in a country as conservative as Britain at least, requires historical incumbency. The libs aren't going away : on balance, it suits the two bigger parties to have a buffer between them.

The inertia in FFP is huge. The historical precedent of the emergence of Labour, pushing the libs into third party status, was only possible because it coincided with the introduction of universal suffrage.

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}

England is not Europe™.

The Green Party has a few patches of support. It has, at least, managed to win one parliamentary seat. This suggests that the Greens have got a better idea than UKIP, that you need to concentrate efforts to win seats in a first past the post system.

However the Green Party has not, so far at least, demonstrated any major increase in support since the coalition was formed. They are not challenging for the status of the English third (and Welsh fourth) party.

I would have thought that UKIP have wider, but thinly spread, support. They also do not seem to be mounting a serious challenge, at the UK Parliament level, to the Liberal Democrats third placed party status.