Iizuka wanted to give Sonic, Tails and Knuckles a third of the game to make it balanced, rather than giving Sonic 10 stages, Tails 5 stages (lazily using same stages as Sonic), and Knuckles 5 stages ala SA1.

>City Escape
Sonic
>Wild Canyon
Knuckles
>Prison Lane
Tails
>Metal Harbor
Sonic
>Green Forest
Sonic
>Pumpkin Hill
Sonic
>Mission Street
Tails
>Aquatic Mine
Knuckles
>Sky Rail (later given to Shadow)
Sonic
>Hidden Base
Tails
>Pyramid Cave
Sonic
>Death Chamber
Knuckles
>Eternal Engine
Tails
>Meteor Herd
Knuckles
>Crazy Gadget
Sonic
>Final Rush
Sonic

Funnily enough Shadow only got 4 stages and 2 of those were reskins of Sonic's stages