Well... here's a theory. I think that Yazneg was the original design for Azog before the film was split into three and the subplot became a more important element as it had to carry across three films instead of two. I think they just realized the design wasn't distinctive enough. Again, this is just conjecture and we'll probably find out for sure on a DVD release some day. But:
1. Actor John Rawls, who played Yazneg, was originally cast as Azog but this was changed during production.
2. The Lego set has Yazneg riding the white warg, and does not include Azog at all.
3. Originally Azog was listed as coming out with series 1 of the The Bridge Direct figures in early solicits, but he didn't end up coming out with them. Yazneg, who was never mentioned and is a VERY minor character in the final film (which would be a little bit of a strange choice for the 6'' line) did.
Again, just a theory and this could be explained in other ways... but... well, it kind of makes sense.