Three reasons, all of which intermix.
The first reason is that the houses are awful. Old, full of asbestos, dilapidated, crumbling - it would cost a lot of money to make Detroit's abandoned buildings useful.
This ties in perfectly with the second reason: liability. These houses are so awful, that if a homeless junkie hurt himself in it, you would be liable. None of these houses are up to code - and ownership means you are now responsible for the condition of the building.
Lastly, high taxes. Detroit is a big city - and its services are paid for by property taxes. But, because more and more people are fleeing the city - there is a smaller tax base. This means that each homeowner has to pay more in taxes to make up for the vacancies. To be fair, it does balance out somewhat - the houses are so worthless that the property taxes end up being reasonable.