Board Thread:General Discussion/@comment-3121860-20170115193443/@comment-679780-20170208225422

My guess is of the following:

Like Finn the Human is regarded as the last human despite his people living on secluded islands outside of Ooo, it could be possible Knuckles was exhiled from Angel Island, the island itself could exist in an alternate dimension or a secluded part of the sky if that makes any sense and thus Knuckles is the only echidna known to live on the ground instead of Angel Island making him unique for his kind.

That or Boom version of Knuckles pretty much failed his duties as guardian of Angel Island, the island fell and is long gone and thus Knuckles is somewhat "adopted" into Team Sonic.

I'm hoping in the future, Sonic Boom will show us places outside of Bygone Island (So far we had on the surface of an asteroid in "Spacemageddonocalypse" and under the sea in "I can sea Sonic's Fear") I would like to see if Angel Island from Sonic Boom is any different from the canon, if it is still there at all.

Before Amy confirmed Knuckles was the last, my headcanon of Boomiverse Angel Island was an island a lot like Themyscira only instead of beautiful warrior women, is full of echidna jocks.