Why did the Black Clover anime stop in 2021?

The end was announced on February 2, 2021: the television series would finish with episode 170 on March 30, and an important announcement would follow. In the event, the announcement landed two days before the finale, on March 28: Black Clover would continue as a theatrical film, the project that became Sword of the Wizard King.

The arithmetic behind the stop is visible in the dates. The anime had been adapting the manga at full speed for three and a half years while the manga, mid final stretch, was heading toward its own pause and magazine transfer. Rather than stretch the story with filler, Pierrot closed the television run mid-arc and moved the franchise to film, leaving the rest of the story in print until the manga itself was finished. When the anime returned as a second season in 2026, it returned to a completed manga.

The stop, in other words, was a handoff: television to film, and film, eventually, back to television.