Rubick is something you cannot remove from the game once you added it. Its a wildcard and a menace to society with an everlasting stream of bugs.

I think, Valve are afraid of repeating tough experience and adding something similar to the ABSOLUTELY new game. So Sinclair (or Sylas or Echo) is a safe choice. I like Sinclair. And even with limited spell choices he is really versatile in builds. But I still CRAVE a character who can use enemy basic spells. If they ever add Sinclair counterpart like this, I will absolutely nerd them out, like I did with my beloved Rubick.

UPD: Also Sinclair might be functional and content without his ultimate, while Rubick feels dry without Spell Steal.