"Communist" countries were authoritarian state capitalist societies because:

1. Marxism explicitly states that capitalism is the required precursor to socialism, which is an objectively false statement, even when considering Marxism-Leninism.
2. Lenin rejected calls to establish a coalition with other socialist parties, and argued that the first elections in the new Russian Provisional Government were not a fair reflection of the people's will and that the Bolshevik agenda was not educated well enough. He opposed the elections, only being overridden by the Council of People's Commissars.
3. The entire Soviet system was based on a bottom-up model of democracy, which is bad for numerous reasons, including that the people at the top are not directly accountable to the people. Voting on candidates was limited to members of the Communist Party, which only allowed new members who were approved by the Communist Party.

Why would all these countries copy a failed model?

Because the Soviet Union had a major role in establishing those nations, and thus had major influence over their government structures. There are of course exceptions, such as Maoist China, however we already have evidence that Mao wanted power.