The reason(s) that TheWrap claim seems pretty legit.
“Spongebob” was originally scheduled for release on Feb. 13, 2015, when it would have served as counter programming against “Fifty Shades,” but it will now hit theaters on Feb. 6, 2015.Source: TheWrap
While “Spongebob” and “Fifty Shades” couldn't be more different, “Fifty Shades” has been described as “mommy porn” and will seek to attract the older female audience that typically takes young children to the movies.