I'm not sure if there's a good answer for this one that is not almost completely opinion based (unless some standards body has published their criteria for creating the current scoring system).
The scoring drives how conservative or risky you can be while still remaining competitive. That reflects one way that the standards-settings bodies for these sports differ.
Compared to organized gymnastics, skateboarding has a tradition of emphasizing new, varied, and risky maneuvers. I think using gymnastics-style scoring would create an environment where the top performers would feel pressure to perform only solid tricks. By having a large number of discards, the athletes instead should be able to select tricks that are closer to the edge of their ability without fearing that a miss will doom their competition.
Opinion time again here. I think that a gymnastics meet where the top few athletes all performed what is the the current "top move" in some discipline and the spectators got to see how each executed it could be a fabulous event.
I think a street skateboard competition where the top few athletes all performed the same trick would been seen as a problem.
