Russia invested a lot in Bulgaria and favored it over the other Balkan states, but Bulgaria chose the Axis and Nazi sides in the World Wars. Bulgaria also reneged on its signed South Stream deal with Russia (though due to pressure from the U.S., and then the EU (which was doing the bidding of the U.S.)). I think they also reneged on their nuclear power plant overhauls too.
Bulgaria is an obedient servant of the west and doesn’t put up much resistance nor protest against it.
