"But I don´t understand why the 2D Sonics couldn´t be made into 3D games without all those problems with the camera. Can anyone here explain what exactly was the conversion problem?"
because of two thing
1 It because sonic and mario aren't the same sonic goes super fast mario not so much and camera don't like fast thing and Shigeru Miyamoto is smart a kepted link, zelda, and mario from talking to much.
2 sega forgot what made sonic last and last going the same level again and again and kept it sill fun, In the same level alone it had 15+ way of going to the end, lot of secet to be found, super sonic, *drooling* and so much more but in 3d all of that geatness gone.