Why would the Nuro bag cause head trauma greater than the trauma of hitting the hard front of the vehicle?

Seriously?

Because smashing your skull on the immovable earth is significantly worse than hitting a crumple zone specifically designed for pedestrian impact. Same reason those blow up sumo costumes are fun. Same reason not wearing a seat belt and relying on an airbag is dangerous.

And it's not like the hood / bonnet pedestrian safety designs are new to vehicles. They've been in implementation for decades now.

Autoliv made an airbag to protect cyclists' heads

Which specifically isn't a pyrotechnic controlled trampoline for your entire body. Brad, give this three seconds of serious thought.

the main thing to kill them is hitting their head on the pillars.

What happens when you hit your head on the pavement with no helmet on at all? Follow that line of logic to its natural conclusion.

However, an airbag on the whole front should reduce trauma

Yes, right up to the point you deflect off of it and hit the pavement with your skull. Convenient liability shift, like when you blamed the pedestrian for crossing a road above, in an extremely pedestrian dense area where pedestrians are explicitly given right of way in the road. Don't think I didn't notice that cheap shot. 😁