Why Dunkerque and Strasbourg were fast battleships

Why were Dunkerque and Strasbourg not battlecruisers?

Because they were designed and understood as small fast battleships, despite their speed and battlecruiser-like appearance.

Dunkerque and Strasbourg were small, modern fast battleships designed around a battleship mission. The French requirement was aimed more at Italy than at the Deutschland class, although the German ships also fitted the resulting threat set. Their speed and relatively modest scale can make the battlecruiser label tempting. Those physical traits do not displace their battleship role, so design emphasis and intended employment matter more than appearance alone.