As a hypothetical, imagine 1 state which has very low taxes, say 5k on 100k income and does not provide very many services requiring the citizens to use post income tax money on all the goods and services they require such as rent, food, and healthcare. Now imagine another state which has super high taxes, say 40k on 100k of income, but the state massively subsidies their rent, food, gives them completely free healthcare and such.
Why should the person in the high tax state get to deduct the state and local taxes used to pay for similar services as the low tax one who has to pay for it with post federal income tax money?
To put it in more economic terms, SALT deductions incentivize state and local governments to increase taxes and spending to the point that the marginal utility of $1 provided by the government to citizens to equal the marginal cost of citizens taxes which include the SALT deductions or (1 / (1 - citizens marginal federal income tax rate)).
That literally creates waste because it incentivizes states and local governments to increase taxes to the point where citizens are seeing equal benefits with overall tax burdens at the cost of reducing the federal governments budget which ironically then encourages them to increase taxes to make up for it which then lowers the denominator in the states marginal utility to overall tax burden calculation in a positive feedback loop.