Why did it bomb at the box office?
It was produced for about $10 million but barely made its budget back!
https://tvtropes.org/pmwiki/pmwiki.php/BoxOfficeBomb/SThroughT
https://bombreport.com/yearly-breakdowns/2004-2/
I wonder if Sleepover was partially hurt by coming out around the same time as Mean Girls and 13 Going on 30, which were edgier and more ambitious but had a similar target audience? Sleepover kind of looked (with its Mean Girls meets Adventures in Babysitting or Ferris Bueller's Day Off meets 16 Candles for the middle school tween set) cheap and cheesy in comparison.
https://en.wikipedia.org/wiki/Sleepover_(film)#Reception