If someone else is cc'ed in then people feel pressure to reply as they aren't just ignoring one person, there's a whole other person expecting a reply. Even if that person is someone they've never met, or know of.
Because in most of those mails it was mission critical that I receive the response. The process grinds to a halt if I don't receive response, irregardless of anyone and anything else. Both Alex and I were tied to the same process and action. The priority of it is unchanged.