I think it's a combination of several factors -
Previously paladin tanks had easy mode AoE threat as mentioned
Buffing synergy was higher between sham/pal then sham/war for a very long time
It seems that it wasn't until a while into Cataclysm that Bear (druid) tanks started to get some of the tools they were missing

If you're liking the warrior stick with it for now, he'll tank just fine down the road. Keep in mind that you can also always start up a DK and catch him up from 60 (or join your team at 60) if the warrior doesn't work out.