Originally Posted by ThePhantasm
Because like the parent's storyline, its a plot point purposely left open.
If they really had no intention of revisiting it, Peter would have caught the killer, (like he always does) in the first film.
However, he didn't.
That should tell you all you need to know right there.
EDIT: Rise_- is right about the camera panning over the wanted poster and picture of his parents during Ben's "Unresolved Things" voicemail. It's a tidbit the vast majority missed but its VERY important because it tells the audience that A) This was purposely left open and B) It will be revisited.