Mordor was released almost 6 years ago. It might be that SSG felt since the expansion release was so long ago that it was not worth it to continue to offer the more costly versions. I would also hazard a guess, that come this August or so, the Standard Edition will disappear from the store. As it might be added to the free list, but that is just a wild A** guess.
Seems silly if you ask me. Cosmetics are timeless and just in this thread you have already few ppl that would buy it and i imagine a lot more in reality. Even if expansion was free i dont see why wouldn't they offer more expensive editions for cosmetics for lowered price of the expansion if its free.
At the end of the day they are a business and they want money. Content is already there so it wouldnt cost them anything they just have to sell it.