Because no one actually cares about "big" or "small" government, they care about using the government to impose their beliefs on other people, whether those beliefs are conservative or liberal in nature. Both liberals and conservatives are actually for "big government", it's just that conservatives also lie about being for "small government". Liberals want the government to take money from the rich and give it to the poor, while conservatives want the government to wage war on drugs (read: ethnic minorities), other countries, women's rights, etc.
But in the end, it's those with connections that are able to skim money off the top of all these transactions and laugh all the way to the bank. Obama allocated $607 billion for defense-related spending last Thursday - $30 billion more than was in the recent budget deal passed by Congress. So actually, the "liberals" are also supporting the military-industrial complex too.
The fact is, when you say you want the government to be small, you are conflating things that there are different. There are different ways to be big or small, and they have different advantages and disadvantages.
You're mistaken - they have much in common. In order to be a successful political party, what you have to do is find issues that (a) a lot of people deeply believe in on an emotional level and (b) can be addressed in such a way that you can redirect large sums of money into the pockets of 3rd parties in the process, who will then support the election campaigns of your politicians.
Whether this happens through providing obscenely overpriced healthcare to Americans or through buying/using drones to extrajudicially murder and terrorize Yemeni wedding-goers is just a detail. We could debate until the end of time how much value either of these initiatives actually provides to the American citizenry, but that's a completely separate issue (and not one that the vast majority of politicians give a rat's ass about).
The libertarian two axis view of politics is still far too simple, but it's a start: plenty of people who rail against "big government" support the drug war, while plenty of people who like redistribution are adamantly against it.*
As long as you don't get that point, you're working with a seriously limited view of politics.
* If you really want to blow your mind, there are people who like redistribution, hate the drug war, and also think the government should significantly decrease its economic regulation.
Edit: slightly nicer phrasing.