I am dying to know it. Maybe something in its name or logo...
You would need to ask the people who are choosing which sites to spam for their motivation, to get any sort of realistic answer.
We can't tell you here, unfortunately, we simply don't know who these spammers are, never mind why they may or may not choose to spam one site over another.
Maybe they like blue things. Maybe they like things starting with the letter 'D'. Who knows :)