DOTP is a period where proletariat seize state power, radically transforming it to pursue their class interests and historic task, that being the end of capitalism as a whole. This class power organizes itself politically through specific organs, one such possible organization being a party that comes from the revolutionary proletariat itself, not something that's politically autonomous.

Soviet Russia for the first few years was a DOTP, Bolsheviks were genuinely hard at work trying to achieve the first pre-requisite for the abolishment of capitalism, that being an international revolution - you can't have socialism in one country. However, those revolutions were crushed, Soviet Russia was completely isolated and the state stopped being a DOTP around 1920, with Bolsheviks starting to play for survival rather than advancement of proletariat interests. Concessions were brought in like NEP, later on it had overgone a Stalinist counter-revolution and national bourgeois interests took a grip.

Tried to keep it brief without going into too much detail, but tldr: USSR didn't fail because le evil cabal of authoritarians, but because the revolution had failed. The ruling class was pursuing economic transformation until it couldn't and the organ lost its class character.