Probably because the income tax is segmented. Everyone pays the same tax on their first $50,000 or whatever. It’s only the higher bracket of income subject to the higher tax. So no one is charged a different tax on the same portion of income.
If they said all your income is taxed at a higher rate if the total exceeds a certain limit, then you’d have a better equal protection argument. That’s when you’re singling out an individual for different treatment, not singling out the income itself for a different tax.
I wasn’t aware about the same tax on the “first” amount of money. What a friggen scam.