It evolved to be flightless because it was useless on an island with no predators, it drowned when the sea levels rose and covered the island, its closest relative (from whose ancestor it had evolved) flew back to the island once the sea levels fell, it evolved to be flightless because it was useless on an island with no predators.
It didn’t.
It evolved to be flightless because it was useless on an island with no predators, it drowned when the sea levels rose and covered the island, its closest relative (from whose ancestor it had evolved) flew back to the island once the sea levels fell, it evolved to be flightless because it was useless on an island with no predators.
It’s evolution all the way down.