there is heavy traffic in that area of the field. backers, safeties, plus the chance that the cornerback jumps the route (which is what happened). by design you have to throw the ball ahead of the receiver in a slant, and you have to rely on fooling all those defenders to not jump the route that is headed to within a few feet of where they all currently are. yeah, it might work, but the risk-reward ratio just doesn't compute.
look, i'm no coach or NFL expert and even my jaw dropped when i saw the ball leave Wilson's hands and where it was headed. that's why my reaction is so strong to this.