The most important factor that allowed Detroit to become the "motor city" was geography. Detroit was simply very well placed to have access to all the things that were needed for the auto industry.
Because of its location in the Midwest and on the Great Lakes, Detroit had access to all sorts of resources. Northern Michigan had ample supplies of iron, for example, and the coal resources of Appalachia were not far away by train. The lakes allowed raw materials to be shipped in and finished products to be shipped out.
Of course, there is a great deal of luck involved. Detroit was not the only city that fit these descriptions. It might well have been the case that some other city would have become the "motor city" if it had not been for the fact that Henry Ford was born near Detroit.