Why can I transmog X weapon into Y weapon but not Y weapon into X weapon?

I had a pickax and a sword on my Fury Warrior, and I wanted to have two pickaxes. But when I went to transmog it, I was unable to make my sword look like my pickax (counts as an ax). I assumed this was because the sword is a sword and the pickax is an ax. But, just to test it, I tried to transmog the pickax to look like the sword, and I was able to.

Why is it like this?

Both of them were two handed weapons and both were soulbound.

Solution 1:

From the WoW forums "Transmogrification Rules".

I think the issue may be coming from

• Main hand weapons can only be used to transmogrify Main hand weapons.

• Off-hand weapons can only be used to transmogrify Off-hand weapons.

• One handed weapons can be used to transmogrify a Main hand or Off-hand weapon.

Was it possible the other sword was an offhand weapon, so trying to transmog a pickaxe -> offhand didnt work?