It's because the opposite is capitalism
I mean, the Nazis considering Bolshevism their ideological opponent and archenemy probably had something to do with this, too.
Other way around. Capitalism exists
because freedom. It's a product of liberalism. The populist mass movements of the 19th century are then in turn a consequence of capitalism.
You cannot get started on capitalism without liberalism (because a lack of liberty results in legal uncertainly and an inability to accumulate capital; nicely illustrated in Edo era Japan, which had exactly this problem, but also in medieval Europe and its sumptuary laws; for that matter, Rome after the Crisis of the 3rd century shows very similar symptoms). In turn, you cannot have socialism without capitalism, because it's capitalism that generates the urban lower class socialism needs to exist as a concept.
Interestingly, although capitalism cannot, as far as we can tell from the historical precedent,
start out without liberalism, it
is possible for illiberal states to nevertheless adopt capitalism (e.g. China). So much for capitalism is freedom.
And this does in turn demonstrate that capitalism isn't a
successor of liberalism, but merely a consequence. A symptom, if you want.
Which also explains why late adopters of capitalism are doing just fine with suppressing political mass movements, whereas liberal states tend to coopt them instead. Capitalism needs liberalism to be born, but not to exist.
And for the love of god, don't lump everything illiberal together as "Basically just communism". That gets you Louis XIV: Communist. Which is retarded.