Commerce: Show taxes on checkout
Taxes are dependent on shipping address so adding a tax line item to the cart does not make sense. Try looking at one of the tax rules on /admin/config/workflow/rules/components to see how the tax line item is added to an order. By default, tax line item is added in the pane that comes after a customer has entered her shipping address and before payment is submitted. Adding a tax line item to a cart requires a custom line-item type and a shipping address to be provided for tax calculation.