I think that the reason was Nissan is not unionized and that would had went against the general rule that also run Detroit; the Union. Many of the empty plants could had been taken over by Toyota, Honda, and Nissan years ago but the former occupants would rather see the plants sit empty for decades than to let the foreign competitors take over the buildings and beat the local yokels at their own games by producing better cars. This is probably the reason why many of the decisions made in Detroit had been ass backwards and not benefitting the city nor the residents of it