Bulgaria was somewhat shielded from the direct force of the Soviet Union, unlike the Baltic countries, Poland, Czechoslovakia, Hungary, Romania and East Germany, which were invaded and conquered by the Soviet army.

Moreover, the Baltic countries were then incorporated into the Soviet Union, together with big parts from Poland, Czechoslovakia and Romania, while the remaining parts of those countries became vassal states, from which Russia continued to steal vast amounts of resources during the fifties, under the cover of some mixed companies established with the locals. In some of these countries armed resistance in the mountains has continued to fight against the Russians for a few years, until eventually all opponents were defeated. In Hungary and Czechoslovakia the Russians made large-scale military interventions in 1956 and 1968.

East Germany has also been occupied by the direct invasion of the Soviet Army, which have also plundered everything that they could, like they also did in the other directly invaded countries. From East Germany, the Russians have stolen entire factories, piece by piece and tool by tool, transporting them in Russia and reassembling them there.

So in such Eastern European countries, the Russians were much more clearly identified as the invading enemies, since WWII until 1990.

Bulgaria had been too far away from the Soviet Union, so unlike most East European countries it has not lost territory after WWII. If you compare pre-WWII and post-WWII maps, you can see that the Soviet Union has moved tremendously towards the West. While other countries have lost much territory, Poland has not lost much area, but the country has moved to the West as a whole, because the Eastern Polish territory occupied by the Soviet Union has been somewhat compensated with territory taken from East Germany.

In Bulgaria, like everywhere else, most of those who had been rich before WWII have been robbed or killed by the communists, but overall Bulgaria has suffered much less during the transition to communism, so I expect that much fewer of them were seeing the communists as external enemies imposed by force.

In all Eastern European countries only falsified histories were taught about the Soviet Union, Russians, WWII and communism, but nevertheless in the countries that had been directly invaded by the Soviet Union there was a large fraction of the population which were aware of the histories of their own families, which typically included the loss of property stolen by communists and relatives detained, deported and/or murdered either by the Russians or by the authorities installed by the Russians. So despite the public brainwashing, it was hard to completely erase the memories of these facts.