I'm inclined to blame ingroup favoritism and the halo effect for this left-liberal talent at getting away with betraying their base (s ee Part 2 ). Also, when the left-liberal base sits on their hands in disgust with their own party, they are often punished with the rapid political rise of sociopaths and nutjobs, like Newt Gingrich's Contract on America in 1994 and the Tea Party in 2010 (though Tea Party has actually turned out to be a valuable ally of sanity on some issues).
If all attempts to hold the less-despicable major party accountable result in the catastrophic ascension of the most despicable wing of the more-despicable party, this is essentially a kind of plutocratic blackmail. The political lesson this blackmail teaches is: let the less despicable party do whatever it wants to you, no matter how vile. It can and will get worse otherwise.
In any case, plutocrats know that outrages against democracy, equality and public health by left-of-center leaders will arouse less opposition than when "right-wing conservatives" execute the same outrages. Thus it serves the interests of plutocrats when we expect too much from left-liberal politicians, or when we have too much fear of right-conservative politicians. The latter can do terrible things, but at least we notice when they do.
Liberal-Conservative Is Not The Same Dimension as Left-Right
So now it's time for an interlude to help you relieve your confusion (i.e. adopt my point of view) regarding the words "liberal" "conservative", "left" and "right." People often take "liberal" as a synonym for "left", perhaps because liberals and leftists often get together politically to take on conservatives and rightists (and vice versa). If you want conceptual clarity, though, it is best to think of "liberals" as hedonistic, open-minded, stimulation-seeking, self-directed types of people and conservatives as devout, obedient, traditionalist, security-concerned people. Liberals like sexual exploration, tweaking their consciousness, watching foreign films, and having scintillating conversations with diverse views expressed. Conservatives like going to church/mosque/synagogue/temple, doing wholesome things with their families, and building up a simple and satisfying existential routine in a stable place they can call home.
On the downside, liberals are somewhat more likely to be psychopaths , drug addicts and sex addicts than conservatives and conservatives are a lot more likely to be homophobes and opponents of abortion rights than liberals. Though it is possible to be liberal and conservative at the same time (and aiming for the best of liberalism and conservatism is a worthy goal), in practice one lifestyle, taken out far enough, tends to become competitive with the other lifestyle for most individuals. A large chunk of people tend to describe themselves as "moderate" or "neutral" with regard to liberalism vs. conservatism, and politicians often pose as being at the center of these poles for that reason. But it there is a wide range of meaningful variation at this "center"--variation along another dimension that is very morally-relevant.
Both liberals and conservatives can be attracted to what I call "the left" or "the right", since this is a potentially independent dimension both conceptually and empirically. What I am going to call "Lefties"--and forgive me for a very idiosyncratic definition of this term--tend to be concerned with benevolent treatment of others around them and a benevolent universalistic system of practices and power relations for all. That means lefties tend to support egalitarian social arrangements, peaceful means of persuasion, impartial procedural justice, care for the environment, restorative over punitive justice, etc. The "right" on the other hand, is tremendously concerned about power and achievement, getting things done, accomplishing goals, and making "greatness" happen--generally their own.
As with "liberal" and "conservative", it is possible to be "left" and "right" at the same time--sometimes a deft mixture of the two will facilitate serious political accomplishment, though the mix can also turn into something ugly fast. " Actually-existing communism " might be thought of as something that began as a powerful combination of left-wing ideology with right wing methodologies and then quickly became a hollow shell of leftism filled with a particularly putrid and vile form of rightism. It is even plausible that a leftist orientation that gets overtaken by methodological rightism leaves a bigger crater in all things just and fair than simple old rightism by itself. Giving leftists God-like totalitarian powers over the lives and deaths of millions of people has a particularly bad record in world history.