Everyone has been saying character, but to me that is a nebulous answer.
For me, the reason is texture. Old structures lend themselves to B&W to me because B&W is about communicating textures and tones, not colors. Old strutures are already faded and worn. The color is usually depressing and uninteresting (though that is at times what the photographer hopes to capture of course), but the tonality and texture is still abundantly there. That is what attracts me. I think when I look for B&W structural subjects, my first inclination is to look for interesting textures or tones that I can capture on film. Older structures make this easy. Plus, they usually aren't something you see everyday, and that adds interest.