They wanted to keep the domain name for branding and legal purposes, but not the association with Magic The Gathering (The domain was registered for a company for trading MTG cards eight years ago, MTG Online eXchange, but it was never used for that purpose)

So, about 2 or 3 years ago, they cleverly rebranded "MTG OX" to "Mt Gox" without changing the domain name.

Then they cleverly lost $500 million.