As has been pointed out already, most tipped jobs have pretty low pay, even with the tips. Some tipped employees would see a benefit. Many would not.

I'm not so sure about that. Median annual income of a waiter/waitress in the U.S. is north of $29K (at least by one source - it varies somewhat) which is comfortably above the standard deduction. Many (most?) of those folks will qualify for the EITC, and many (most?) will have an EITC that's higher than the amount of taxes they owe - but exempting tipped income from taxation would still benefit them.

money.usnews.com - Waiter and waitress

I'm pushing back on this not to be pedantic, but because I think that Democrats are still reluctant to see the threat that Republican populism poses for their coalition. The corporate/fiscal conservative faction has lost most of its ascendancy that it enjoyed as part of the Reagan (and Thatcher) realignment. It was a nice forty years while it lasted (for them), but right-wing parties all over the West have moved away from austerity (or even faux austerity) programs into full on populism. Democrats don't want to believe that this new populism is real, because if the GOP can pivot away from the Grover Norquist/Paul Ryan "let's cut all the popular governmental services" wing of the party, it's hard to see how the Democrats win elections.