Most of the debate between different political ideologies (as opposed to “our guy is better than your guy” debates) occurs between liberals and libertarians. The ideological debates that pit conservatives versus liberals or libertarians come in a weak second.
Why is it that the fiercest debates occur between liberals and libertarians? It is not out of any leanings towards conservatives that these debates occur. Conservatives allege they share economics with libertarians but a closer examination proves otherwise. Conservatives are quick to steal libertarian rhetoric but fail to follow through on it. The real reason there is so little debate is because there is so little to conservatism. Mercantilism isn’t a school of economics as much as it is a way for people to use the government to line their own pockets and the rest of what is commonly called “conservatism” is a morality platform based loosely on religious concepts. That is why Jerry Pournelle listed conservatism as irrationalist, while he listed both liberalism and libertarianism as rationalist.
Liberalism and libertarianism spring from the same root, classical liberalism. One side emphasizes liberty, and the other equality. Both see the other as essentially missing the point, which is somewhat ironic given that essentially the two are trying to achieve the same thing but by entirely different methods. The programs suggested, the laws sponsored, are opposed to each other to achieve what is ultimately the same goal. Liberals and libertarians both have their root in the enlightenment and try, in different ways, to seek justice. One seeks it through equality, the other through egalitarianism.
That is why there is plenty of room for debate. But that is not a full analysis of why there is such fierce debate between liberalism and libertarianism. Another reason is that many who would be called liberal are instead progressive. Libertarians and liberals pursue opposite policies for identical goals. Liberals and progressives pursue identical policies for opposite goals. Progressivism doesn’t share the Classic Liberal root of both libertarianism and liberalism, but instead uses the tools of liberals for their own ends. Since they advocate the same policies they seem to be the same.
There is no common ground between progressives and libertarians, other than also both being listed as "rationalist" by Jerry Pournelle. One seeks justice through liberty. The other seeks control through dependence on the government. The programs advocated by liberals are precisely the tools progressives can use because the argument can be easily made that the programs are there to help the recipient. And in being helped, they become dependent until the progressive achieves his goal: everything in the State, nothing outside the State, nothing against the State.
One argument is a civil war. The other is oil and water. In both cases, unlike the conservative, there actually is substance though. And the progressives do worry every time the phrase "liberaltarian" comes up or liberals and libertarians find common ground, because if the two children of classical liberalism were to ever rejoin the progressives would be through.