Why was my article deleted?

Copyright isn't the issue. If he wrote something for WP or RW he agreed to release it under the relevant version of the CC-SA license.

Thank you for explaining that, I had only a foggy idea of what was going on.

And by posting it on CP, he dual-licensed it under their provisions. As such the source still matters. CP could have added additional changes, or he could have created a derivative work himself on the CP page, and we and the rest of the CC-SA license sites would not be privy to those derivative parts.

Thus the point: if it followed through CP, it's tainted.