There are a myriad of reasons... but my best guess would be because the Bitcoin crowd is
currently a very niche group of individuals. Kickstarter works so well because even your little sister could donate a couple bucks to a project with mommy's debit card. In the real world (i.e. outside this forum), most common-folk might know what Bitcoin is, but haven't used it or are very suspect of it..
Regarding the current Bitcoin crowdfunding sites, it isn't necessarily about being profitable at this particular moment. It's about being first to market