Why the hell was there an election in Zimbabwe when Morgan Tsvangirai pulled out leaving ONLY ONE candidate?

The whole thing was a complete shambles. I don't understand how there are some people defending it & calling it a legitimate election process either. Did Morgan back out mid-election or something? Either way, the whole government in Zimbabwe (& half of Africa for that matter) is highly corrupt so I'm sure Mugabe would have found some way to win whatever happened.