Not only that but a new vector is like a gold rush. It's a virus infected a new host that hasn't developed an immune response.

A new vector (probably) hasn't built hardness yet against spam, like for example email with apps like Gmail filtering out most spam and SPF etc.

If you start a forum, you expect spam, and there will be tools in the forum software to deal with it. But something new that isn't a traditional place to spam may be caught off guard.