I honestly don't know. Just a few months before the Femme Fatale era, Britney gave Glee one of their biggest audience impression. I simply think people are interested in Britney Spears the pop star and her work as opposed to Britney Spears the judge of talent. Considering Britney is already infamous for lip-syncing maybe people didn't take her seriously enough to watch. Then there's the fact that since last season was far from amazing, people might of thought since last season wasn't the best with Paula and Nicole that this season wouldn't be any better. Quite honestly, I only watch this season for Britney and if she wasn't on it I wouldn't give the show the time of day.
Then there's The Voice which many people consider to be more interesting and the judges have better chemistry so that show could be raking viewers away from X-Factor.
But no need for more excuses. If Britney isn't the ratings juggernaut we thought she was, I can live with it.