Because people want to blame somebody other than Harry himself for his utter passivity and lack of curiosity. A lot of Dumbledore and Hermione bashing is also based on the same thing.

Also, to be fair the "normal teenage" explanation for any of the golden trio member's behavior really doesn't hold up, because they never had a normal teenage life in the first place to be acting like regular teenagers.

 

By 'normal teenager', I'm assuming it means 'normal teenage emotions' that would influence the choices made. For example, Ron's normal teenage hormones at least somewhat influenced his...poor choices in DH. The way I see it, their not-normal teenage life causes them to have to make these choices and their normal teenage emotions causes them to make a certain choice.

 

It's a minor detail in their larger failure to do her justice, but I think fanfiction tends to exaggerate Luna's discussion of imaginary creatures in her dialog. That's obviously there in canon, but I feel like the weird conspiracy theories she also spouts in the books get short shrift in comparison. And also, some of those conspiracy theories are relatively mundane. Stubby Boardman, Scrimgeour the vampire, they don't make sense, but they also don't require you to be on drugs to understand them. I think those things get lost in most fics featuring her.