It's summer. Families with kids need stuff to do. Everyone already knows the character. It's voiced by the incredibly popular Ellen DeGeneres. This was a no-brainer.
And because Disney let the actual Pixar people write it, it's also very good (according to the reviews). Here's one from the Wall Street Journal...
What was a fertile comic notion in the original—Dory’s short-term memory loss—is now at the core of her seriously affecting, though sometimes repetitive, search for her parents. This sequel redefines the notion of loss leader. Dory lost track of her mother and father long ago; Marlin and Nemo, having lost and found each other, help her on her quest. Still, the movie’s main source of energy is screwball comedy. “Finding Dory” can be touching, sweet and tender, but it’s compulsively, preposterously and steadfastly funny.
