Great songs flop/fail to permeate the public consciousness all the time, and **** songs (e.g., Roar) smash just as frequently. Merit is only one of a few factors.
apparently they were between it and another single (probably WOA) but radio started picking birthday up so they're going with that. if it dies down quicker than they expect, WOA is getting released but WOA won't be getting a release at all if it doesn't.