I think I know why Sonic Lost worlds is called that

[cuberandgamer]

wfz said:

In the IGN interview that went out earlier this afternoon, they said that they were building the levels completely around the gameplay opportunities, not the other way around that they usually do. This means that it's extremely unlikely that these levels at all represent cancelled game levels.

It's simply called "Lost World" because the game takes place in a lost world that Sonic encounters while chasing Dr. Robotnik.

http://www.ign.com/articles/2013/05/28/sonic-lost-world-taking-a-page-from-mario-galaxy

Thanks for sharing :D