The only reason I could think of is Britney has a new album to still promote and Rihanna is about to move on to CKB. But I agree, any new Brit single and CKB need to take a backseat to this for now!
This song was never suited for Britney vocally....but she did well. Britney SLAYED the bridge....and Britney's verse is hot lyrically. She even slayed the 'COOOOMMEEE ONNN' --- but Britney still has a............specific voice, and for a song where you really have to SCREAM IT, she couldn't do that.