Effect of specific tax on demand and supply
The answer has to do with the fact that (under typical assumptions) producers experience increasing marginal costs as output rises. This makes the supply curve upward-sloping. 

Suppose firms simply increased the price by the amount of the tax. Consumers would then buy fewer units of the good, resulting in less profits for producers. However, marginal costs are also lower at the lower level of production, meaning that firms could profitably produce at a lower price than that. So they lower the price and increase production a bit to partially offset the losses caused by the tax (note, the price remains above where it would be without the tax, and production still less than without the tax).