But I never understood why the old Liberal Party began to decline to begin with. If anything, you'd think they would have been a more logical place - in terms of ideology - for supply side economics and neoliberalism to grow in the 1970s and 1980s than the Conservative Party would have been.
I think a better question to ask is why the liberal parties in Canada and United States didn't decline. After all, the patter in most of the west is a conservative party versus a labour party, with other ideologies bringing up the rear.
Liberalism in the way that the original Liberal Party was liberal (free trade, extending the franchise, etc) is just part of what America is. Our Left and Right parties are just extensions of that premise. Old Toryism would require existing institutions in need of defending - there were/are none here: no peerage, no Church, no monarchy. Socialism/social democracy requires a sense of community cohesion that our individualist society of temporarily embarrassed future millionaires lacks. And because those things weren't there, we never had any major issues with fascism or communism, which is why we also lack a European-style center-right/Christian Democracy presence.
We're Americans. The rich want as much money to stay in their private bank accounts as possible, consequences to society be damned. The poor want as much money to flow into their private bank accounts as possible, consequences to society be damned. We're not interested in tradition or class or national identity or the well-being of the community. And the rest of our politics is determined by how we feel about fetuses and guns.