For your first question; Because no other Nameks constantly train like Piccolo. Not to mention; Piccolo has not only fused with the strongest Namek that was on Planet Namek, but also with Kami. After fusing with Kami, Piccolo became a “Super Namekian”. Even surpassing SSJ1 for a time. read more
There are no other Namekians as strong as Piccolo because (A) none of them fused with each other the way Piccolo, Nail and Kami did, and (B) Piccolo was a “Super Namek”, whatever that means, so he’s stronger than the others. read more
It is unclear if this ability is related to King Piccolo's Demon Clansmen status as no other Namekian has ever been shown using it, not even Piccolo (who lost his Demon Clan status by the time he killed Raditz and Goku as both went to the afterlife as souls killed by Namekian Demon Clansmen remain in limbo). read more