The SALT deduction acts as a subsidy for state and local taxes - reduces their after tax cost. If you're in the top tax bracket, any amount paid over the $10,000 cap acts like a 35% state and local tax increase for the amount over $10k.
Ex. Pay 100k in SALT, take a deduction for the full amount, gives you a $35,000 reduction in your federal income taxes paid/payable.
Post TCJA, you get a deduction for $10k only, saves $3,500 in federal income taxes.
Difference - $31,500, which is equivalent to a $31,500 tax increase, because that's the amount your federal income tax bill exceeds what your bill would have been under the old law.