High traffic areas are higher risk areas, whether controlled or uncontrolled airspace. Midair collisions occur in both: fixed-wing/fixed-wing; rotary/rotary; and combinations thereof.
The crash occurred at something like 8 nm into the Piper's flight from Teterboro to Ocean City, an extremely limited time to monitor traffic, self-announce as one exited a controlled airport, and merge into a very busy VFR corridor. Not impossible or unreasonable, but challenging and unforgiving of conflict.
It also appears to have been less than 2 nm from the helo's departure pad. That is not a lot of time to update traffic and it's too much time for a helo's pre-departure clearing turn information.
My best guess is that neither pilot saw the other aircraft at any point. The natural inclination is to direct most of your attention to the front, where you're going, and where the hazards close quickly with you. Looks to me like the Piper was turning to follow the published route at the point of impact.
It's hard to visually pick out traffic against visual clutter, even with electronic aid. If the traffic is low in your perpendicular, and with little relative movement, it's really hard. I'd guess that was the Piper pilot was pretty busy as he chose to enter a high risk environment. No blame intended by implication...
The helo was probably still climbing and establishing on the route, coming from one risk environment into another and the airplane approached in his perpendicular and blind side, perhaps from beyond his visual range from his clearing turn. My experience is that I'd be 2-3 minutes into the flight when I got to what I'd guess was the collision altitude. The airplane could easily have been out of the helo's visual field when the pilot was looking in his direction. My TIS is often not even updated or downloaded at this point, so much for that.
A position or controller's call would easily be missed or covered, and the stars are aligned for tragedy.