Because I know you looked at my post in the Supreme Topic of Other Knowledge: The fun prototype of Sonic 1. With a higher resolution screenshot, we can see that the old background isn't so much "different" as it is entirely unfinished. There's a straight line cut-off. This explains why we have screenshots with seemingly no background - they're not two different builds, they're just highlighting the fact the background doesn't scroll vertically. Quite the oddity, because frankly the prototype background looks better than the final. It's more interesting, and seems to suggest multiple layers of parallax scrolling were intended. Otherwise it would look like garbage. But get this, on levels which can wrap vertically, there is only ever one background layer. You see it in Labyrinth Zone, and you see it in Scrap Brain Zone act 2 (and 3). For whatever reason, a fancier, pseudo 3D background effect couldn't be done. Maybe it was a design choice, but the final Labyrinth Zone background is hardly thrilling. A stumbling block for a young Yuji Naka? And question: did they give it another go? Hidden Palace Zone opts for a similar background design to the above prototype, and yet its layout suggests it was meant to have a vertical wrap-around. People have suggested it was the inclusion of water that ruined their dreams... but Labyrinth Zone Act 3 has water. Was it the parallax scrolling that killed it? Incidentally Metropolis Zone, the only Sonic 2 stage to have vertical wrap-around, also only uses one background layer. It doesn't appear they solved this issue until Sonic 3.