The reason why liberalism is vulnerable to leftism is the same reason as why conservatism is vulnerable to liberalism and leftism.
If the sacred book of your religion says an absurdity like «We hold these truths to be self-evident, that all men are created equal», leftism is the logical consequence of this belief.
For example, if people are equal, then the different ethnic groups should have the same incarceration rate. If this is not so, the only explanation is that some ethnic groups are unfairly incarcerated. Black Lives Matter is only the unavoidable consequence of the Declaration of Independence.
If you believe in this dogma of equality and your entire civilization believes in this dogma, you cannot defend from leftism. Every attempt to defend from leftism implies the rejection of this dogma and this rejection is taboo, verbotten, unthinkable. «You are literally Hitler».
So people try to defend from leftism while holding this dogma with convoluted arguments, which is an incoherent and weak position so it is doomed to failure. You can write a book defending the current US prison system and the only thing that a leftist needs to defeat your ten years of study is a word: «Equality! Non discrimination!»
Conservatism and liberalism are only two ways of dragging your feet on your way to the logical consequences of equality and other absurdities, which your civilization assumes as self-evident truths.
The difference between conservatism and liberalism is a bunch of «unprincipled exceptions» (http://www.amnation.com/vfr/archives/005864.html), exceptions to the dogmas of your religion. Conservatism has more unprincipled exceptions than liberalism, but every unprincipled exception is logically indefensible and, hence, it is doomed to be eliminated when your religion naturally develops to its logic consequences.
This is why conservatism is replaced by liberalism and liberalism is replace by leftism. Nothing to see here. Move along.