Trivia: Sonic makes a brief joke about the feud between the actors Vin Diesel and Dwayne Johnson, who both appeared in the "Fast and Furious" movie franchise. This film is from the same producers as that franchise.
Trivia: Sonic not being able to swim is a callback to the original video-games, where Sonic can't swim. When he's underwater, he can only movie like normal (albeit slower), and will drown after a brief period. In order to avoid drowning, he has to either surface or make contact with large air-bubbles that he can breathe in. This is also referenced in the movie when Sonic sucks in a giant air-bubble in the temple ruins while he's underwater to avoid drowning.
Trivia: Idris Elba has stated that he pitched the idea of giving Knuckles a high, squeaky voice to humorously contrast with his strength and fierce persona. Thankfully, Elba and the director decided that this idea wouldn't work, and would probably just annoy fans, so they instead settled on a more tough, regal voice.
Trivia: Tails is voiced by Colleen O'Shaughnessey. This is actually notable, as she had previously voiced the character in several video-games and the animated series "Sonic Boom!" She's the only voice actor from the video-games to return to voice her character in the movie series. The rest of the characters are voiced by celebrities.
Trivia: There is an extra scene during the credits that introduces the character Shadow the Hedgehog, a popular character from the games.
Trivia: At one point, Knuckles hits Sonic and Sonic drops and loses his bag of rings. This is a subtle reference to the video-games. In the video-games, Sonic collects rings which act as a sort-of surrogate health-meter. When he gets hit, he loses all his rings, and has to recollect them. (If he gets hit again while having no rings, he dies).
Answer: Because Robotnik was betraying him. Dr. Robotnik promised to help Knuckles get the Master Emerald, so they could use it to defeat Sonic. But Robotnik was using him all along because he wanted the Master Emerald for himself... he never actually cared about Knuckles and never intended to help him get the Emerald. Thus, Knuckles is shocked and says that Robotnik taking the Emerald for himself was not part of their deal.
TedStixon