Why is my federal tax deduction for state tax of over $6,000 only $602?

Turbo Tax says that a sales tax deduction  is better at over $1,000, but never shows or explains how the deduction for over $6,000 in state income tax is calculated. I can't find a way to calculate it on turbo tax or check it. It seems illogical that over $6,000 in state income tax is not worth more than $602 in deductions.