Given the poor track record of DCEU movies on Rotten Tomatoes James Gunn wanted a fail-safe movie that doesn't get bashed by critics and he succeeded.
But in turn he made the the movie forgettable at best. So many characters stuffed in such a short 2 hour window. Silly CGI, no proper background story and a weak portrayal of Superman.
No wonder it couldn't even beat the collection of Man of steel.