Siakim is an interesting player, and I would love to have him on the Kings. But does anyone believe that Siakim has superstar potential? I don't! I think he's being played to his strengths, which happen to compliment those of his teammates perfectly. Put Siakim on the Suns and how good will he be? Would he be able to have the same success when he's the center of attention of the other teams defense.
To me, that's the difference. I think Bagley can play on any team in any offense anywhere and still succeed. It took 4 years of professional ball for Siakim to reach the level he's at right now, and that level has already been reached to some degree by Bagley in his first year. And Bagley has been the center of attention of the other teams defense. He regularly gets doubled, and has a hard time sneaking in for a backdoor. Siakim doesn't get that same attention.
None of this is a knock on Siakim. I think what he's accomplished has been remarkable and more power to him. But to my mind, Siakim is a role player that performs that role like a star. He knows what he can do, and what he can't do, and sticks to what he can do. And what he can do, is pretty dam good.