The difference is that liberal democracy is underpinned on the idea that being able to elect a bourgeoise representative is all you need to be fully involved, whereas a socialist system must recognize that collective ownership of a state by the people requires the people have power over everything that happens in that state, law, economics, religion, war, everything. Socialist states exist with this as an ideal and only walk back from this goal with good cause, as opposed to starting with nothing, adding the opportunity to choose bourgeoise representation out of a small pool every once in a while, and calling it good.
e: added text in italics for clarity
Whataboutism is when a leftist proves a liberal wrong