I don't know any reason why not playing a druid tank.
The differences between warrior and druid tanks for 5 man instances will be minor I think.

Warrior will take less damage, because of defense stance and shield block.
Singel target threat, druid is stronger.
Aoe threat both have problems, can't really tell how bad aoe threat is for druid.

You can also consider to use your warrior in your team and boost your team with the warrior through instances.
I did the same with my warrior and my WMMLS group.