There never were socialist states.
No liberal democracy hay ever become (permanently) socialist – every attempt at democratic socialism has so far ended in a rollback forced via IMF-monitored “stabilisation programmes” (Portugal), a coup (Allende’s Chile), an assassination (Lumumba), or successful preemptive counter-propaganda (Corbyn, today’s Chile).
But no post-revolutionary state under “really existing socialism” has been socialist either. Any other assessment is a misreading of history.
In the early 20th century, the revolutionary party (or rather “party form”) as implemented by the Bolsheviks is the most successful (form of) revolutionary organisation.
Its framework of “democratic centralism” is experienced as an effective form of social organisation that combines hierarchy for effectiveness (“unity of action”) with representation for legitimacy (“freedom of discussion”).
Within that framework, the representation of opinion (interests are thought to be aligned via class membership) is secured within the party if the discussion is indeed open, i.e. if it is accessible for everyone and gives equal chances to be heard and followed to everyone, and if unity is guaranteed for any decision, regardless of its originators/sponsors.
This means the system is fair if everyone has the chance to be in the majority, so they can accept to be in the minority. During the formative years of the Bolsheviks, a set of principles and practices is developed to make sure this is the case. Once they are implemented party-wide, factionalism and extra-party organisations are not tolerable because they would undermine them and thus the fairness and effectiveness of the system.
The goal of a national (or otherwise geographically bounded) revolution is to capture state power and extend this system of organisation to the “revolutionary” state.
This extension is meant to be an instrument for an ongoing revolution: Socialism (which means the socialisation of the economy) and finally communism (in which the state “withers away”) are only achievable globally, so the revolutionary nation state is meant to promote and support a global revolution to achieve socialism – and not to be socialist itself. (It can also be thought of as a necessary stage in a dialectical process that will culminate in the final synthesis of stateless communism.)
But then democratic processes get captured by “hidden” factionalism, Power structures, and personal influence. A key reason is that due the unitary nature of the system, there is no externalor power structure to keep this in check (as opposed to in a liberal-conservative “checks & balances” system).
The mirror structures of state and party further reduce accountability and provide additional entry points for power accumulation and corruption.
More generally, society is too complex for the organisational model of the party and democratic centralism, i.e. the model is scaled beyond its usefulness. In the long run, this leads to rigidity and brittleness of the system, stunted social development and economic underperformance.
And most importantly, the global revolution doesn’t happen.
Thus the post-revolutionary societies are stuck with a revolutionary organisation – an organisation that is “adapted” to the new circumstances by being captured and exploited for personal power and gain or group interests.
Revolutions try to force systems into imaginary attractors, but in the case of a Leninist revolution the system instead settles into the Attractor state of an autocratic oligarchy controlling a planned economy in the form of state capitalism. (The Ideology of “socialism in one state” is just a fig leaf for this reality.)