They’re substantially larger and heavier

Size and weight. Combined with "thrown haphazardly into the air".

With a normal dart you poke an eye out. With lawn darts you poke a heart out.

Also many darts for kids these days are plastic.

They're a foot long, and the metal tip is 1/4 inch thick.

Also, I think you're supposed to play with everyone standing on the same side and throw the darts at a circle on the ground. However when I was a kid, everyone I knew played by standing on opposite sides of each other and dodged the darts as they fell from the sky...

It’s difficult for a child to throw a normal dart through a friend’s skull. But this has happened with lawn darts.

With normal darts you kinda need to try to intentionally harm someone. Not that kids won't do that. But still the sector of risk is smaller. Big much less controllable darts on other hand might land in general area near someone.

To give normal darts the damage potential lawn darts have you have to:

   1: cut off part of the plastic tail so that the whole dart fits in a ~1m piece of PVC electrical conduit
   2: wrap electricians tape around the brass front so it fits snugly but not too tight in the same piece of pipe
   3: put the dart, point forward, into the pipe
   4: connect the back (or "breach") part of the pipe to a large foot-operated bellows as used to blow up dinghies.
   5: aim the pipe at a brick wall
   6: jump, or have you accomplice jump on the bellows
   7: walk towards the wall and extract the dart from a brick into which it has embedded itself about 1.5 cm.

Source: personal experience. We did not have lawn darts. We did not need them. Even at that age we were smart enough not to aim the pipe upwards to see how high the dart would go in the knowledge that it would eventually come back to ground level.

WW2 era bolt action 22LR for my 12th, but it was never considered a toy