Why was my article deleted?

By writing it for CP you release it into their copyright, just like if you write something here you release it into our copyright.

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.