Those pants are restricted to Demon Hunters, and at this point the developers haven’t yet enabled the transmog to be learned by anyone other than Demon Hunters. All the tier gear from Legion and Tiers 1 and 2 from vanilla are like that: they drop the actual gear, it’s class-specific and soulbound, and therefore cannot yet be learned by anyone who didn’t loot it.
The devs will change that sometime after TWW launches, but they haven’t gotten to it yet.
Have patience. If you can spare the storage space, hang on to those and they’ll become retroactively available for learning transmogs.
Otherwise, expect not to learn the class-specific tier items from Tiers 1, 2, 19, 20, and 21.