As others already mentioned, not every Warrior can tank. We needed a Warrior who can peel and initiate, Zarya does none of that.
If I were to split Warriors into categories:
- True solo Tanks: Muradin, ETC, Diablo, Stitches, Johanna (They are perfectly capable of being solo Warriors in pretty much every composition)
- Niche solo Tanks: Tyrael, Anub'arak, Varian, Cho'gall (Can be solo Warriors in very specific compositions, also great secondary Warriors)
- Main Tanks: Arthas, Garrosh (Weak as solo Warriors, can be played as Main Tank in any double Warrior compositions, Arthas is also great as Bruise/Off-Tank)
- Bruisers/Off-Tanks: Sonya, Artanis, Chen, Dehaka, Leoric, D.Va (Unable to solo Tank, need to be paired with one of the above)
- Niche Off-Tanks: Zarya, Rexxar (Unable to solo Tank, weak in general but strong in very specific situations)