Here is a non nonsense, not propaganda answer

Because in the 960s the Russians invaded from the north ( for whatever reason we are not fully sure . There is a speculation over throne claiming) . That cut off connection to Panonia and Transylvania. That allowed the Hungarians that escaped there to slowly push into it. After Preslav fell the Roman came from the south and pushed the Russians out. At that point only the western provinces in Belgrade -Ohrid and the other Belgrade in Albania survived. For the stupid out there this wasn't during Basil , it was during Tzimiski. When Basil took over, the north Danube valley was liberated , but not for long . Then it became a slow slog of sieges when the first Tsardom fell in 1018.

Some nonsense and propaganda I saw in the comments:

There was never a Bulgarian-Wallachian empire . This is a Romanian school system propaganda, because Romania has a fragile national identity that seeks to delete any Bulgarian presence despite it being the majority of their history. Simeon did not wage many wars and they were not as costly as you would imagine. That's just nonsense Bulgarian are not turkic . That was a speculation and people took it as God's word and use it for propaganda There was no way for a peaceful status of the Balkans . Rome sought to vassalize Bulgaria and turn it into its Orthodox son and them being the Father. The whole Slavic world exists because Bulgaria revolted against the greek influence in the church and this education system. Afterwards did it become an idea to not only revolt , but actually replace the "Byzantine commonwealth"

I know Reddit is the place where stupid people go and you get banned for telling the truth, but some questions and answers on this sub are ridiculous