Which is incorrect, crit increases parry.
I would need to see a snapshot of your specific case. Typically, all four secondary stats are decent for most tank specs, so sometimes talents, your other gear, etc. will tip the balance towards one or the other. While crit does give parry, haste gives you more resources, mastery gives you better blood shields, and vers gives you straight up damage reduction. In other words: all the stats have a defensive benefit. They added the crit->parry conversion so that crit wouldn’t always be worse than the other defensive stats, not so that it would always be better.
You can make a snapshot by pressing the “help” link next to the big Best in Bags section header, then click “Create Support Post” – you’ll see the snapshot ID on screen and can copy it here.