Do you understand what DPS stands for? It is: Damage Per Second.
When Pokemon defend, they do not constantly attack, like you can do while attacking. Instead, there is a ~2 second delay between each one of their attacks. Therefore when defending, DPS is much lower than when attacking. Low damage fast moves are especially bad when defending, while high damage slow moves are the best, because it minimizes the defending Pokemon is 'waiting' between attacks.
TL;DR summary: When defending, the best moves are simply the ones that do the highest total damage (i.e. Bubble).