Why Scharnhorst avoided slower battleships

Why did German fast battleships run from slower R-class battleships?

Because winning could still leave the German ships crippled, unable to catch the convoy, and unlikely to reach home.

Scharnhorst and Gneisenau could defeat a single Revenge-class battleship together, but victory was not the only calculation. The R-class ship could inflict enough damage to prevent one or both German ships from returning home. While the capital ships fought, the convoy would scatter, leaving the raiders to spend time finding and overtaking merchants. British reinforcements would also be racing toward the area and could defeat the damaged Germans. A tactically winnable battle was therefore a poor exchange if it sacrificed two scarce capital ships without destroying the convoy.