Why did the first Bulgarian empire fall under Byzantine rule in 1018?

The fall of the First Bulgarian Empire in 1018 was the result of a persistent, multi-decade military campaign led by the Byzantine emperor Basil II, who sought to reassert imperial authority in the Balkans.12

Following a period of internal instability and external pressure from various groups, the Bulgarian state was revived under Tsar Samuel, who established his capital at Ohrid and expanded his influence across the region. From 986 to 1014, Byzantium and Bulgaria engaged in intermittent warfare.321

The turning point occurred in 1014, when Basil II won a decisive victory over Samuel’s forces. In a display of extreme ruthlessness, Basil blinded approximately 15,000 captured Bulgarian soldiers, leaving one eye for every hundredth man to lead the others back to their Tsar. The shock of this event is said to have caused Samuel’s death. Following this collapse of leadership and continued military pressure, the remaining Bulgarian resistance was crushed by 1018, leading to the total incorporation of the Bulgarian state into the Byzantine Empire.312