For one thing, it's the only stage with a large pool of water that Big doesn't go to. Also, I've always found it odd that Big's "house", where his opening cutscene takes place, is in the Mystic Ruins jungle, but his game begins in Station Square. Lost World would likely have been his first level, as it's practically right next to his house, and looks like exactly the kind of place Froggy would run off to. I can certainly see Big blindly chasing Froggy in there, not paying any attention to the dangers within. One more thing: you may not have noticed this, but every stage is visited by exactly three characters—except for one. Lost World is only visited by Sonic and Knuckles.
I can think of a couple reasons why they would have removed this. The first one might be obvious: that pool is huge. Finding Froggy, let alone actually catching him, would be a nightmare. Another thing is that it could have been too hard to make fishing from a moving platform (the snake) work, especially if you have something hooked. They could have removed the snake from Big's version, but it would be hard to justify why it was there for Sonic. Also, the pool acts as a bottomless pit. When you reach the bottom, you die. Even if you didn't, there's no way back up, so you'd drown anyway, similar to the infamous spike pit in Mystic Cave Zone. Sure, there's the Life Belt, but if this was indeed supposed to be Big's first level, it wouldn't make sense for it to be an upgrade, since you'd need it as soon as you start playing. One more thing: even four levels is a lot for Big. Maybe the developers realized that—thank goodness it's not five.
So what does everyone else think? How likely do you think this is?