Primarily because the Libertarian Party, as it is in the US isn’t simply about individual liberties. They’re also for pretty much completely unregulated corporate liberty, and they tend to reject the notion that government should play any role whatsoever in a) regulating markets or b) doing anything to ensure workplace safety, ensuring minimum compensation, or similar worker protections or c) doing anything at all to require even basic compliance with things like the Clean Air & Water Act or similar environmental protection.
The Liberterian Party as it is in the US is generally opposed to even the existence of things like the Clean Air & Water Act, on principle, as they’re opposed to things like legal protections for workers, again, on principle. Their notion is that unrestricted capitalism is going to deal with all of these things by itself, and that companies that mistreat workers will end up being companies that go out of business, because folks will just choose to work elsewhere, or customers will purchase from other companies, etc.
The party as it stands is opposed to any notion of antidiscrimination legislation. Don’t want to hire queers? No problem – you’re the owner of the business, and your liberty trumps any other concerns. Don’t like the idea of employing women? Hire all men. Think folks should be paid a per-piece wage? No problem. Get rid of pretty much the entire social safety net? Let the market do its thing. Eliminate public education? Yep. The market is magic, again. The market will do its magic and everything will work itself out “in the end.”
The answer to just about any issue, for libertarians is “the market will fix it.” It isn’t really a coherent political philosophy – it gets dressed up as one, but when you break it down, the answer is always “the market.” It’s an article of faith that the market can fix, well, everything. The reason liberals distrust this is because there’s simply no good evidence that an unregulated market the best possible solution – or even, in some cases, any possible solution, for all of the various things they’re claiming.
Liberalism fundamentally says that there is such a thing as a common good, and that government and business need to directed to serve that common good – in the case of businesses, not exclusively, but sufficiently. Some individual liberties which aren’t in conflict with a common good are pretty great – it doesn’t harm me in the slightest if you criticize liberal ideas, so individual liberty where speech rights are concerned is a thing that libertarians and me, we agree on.
But libertarianism says that there’s fundamentally really no such thing as a common good – there’s just individual liberty (and the accompanying corporate liberty) uber alles. Liberals – and conservatives, for that matter – disagree. We’re saying that individual liberty matters – but collective good also matters, the things the society as a whole values also matter (conservatives and liberals tend to disagree what those common values ought to be, but we generally agree that societies ought to have some of them). Liberterianism rejects any such claim because it might interfere with somebody’s claim to personal liberty.
Most liberals – the vast majority, including this one – see Liberterian positions on corporate liberty to be either a) hopelessly naive (it’s simply not a given that unregulated capitalism can work they way they say it will, or that everybody will just go work for somebody else), or b) profoundly dangerous. While I agree with some libertarians on some social issues, there isn’t remotely enough overlap for me to ever consider joining the party, as it currently stands, or voting for a libertarian ticket.
That’s open to future revision, if they change their minds, though. I just don’t think it’s likely.