I have no idea why. Your gear is adequate for heroics. More stamina will come with more gear - but you already have some decent bits and pieces.
Work on your Wyrmrest & Argent Crusade reputation. You could do with a head enchant and the gear pieces from Wyrmrest will help you.
In terms of enchants, you should start the Sons of Hodir dailies, for the shoulder enchant, you're missing a shield enchant - 22 def there - and also I guess you can upgrade your boots to 22 stam if you want to eke out every last.
And priorities - perhaps run the easier heroics - VH has some irreplaceable (before naxx) legs for you and Stratholme has the shield.
The only thing I'd say urgently needs a check is that you're not specced for Incite - its a massive threat boost for 3 talent points - I really suggest you go for that.
Hope that helps.