Because government taxes and fees are a constant rate (given a particular good and location, anyway).
If I'm price-comparing hotels, and I see one that's $100/night, and another that's $75/night, I know that the cost may be higher due to taxes, but it will be higher by the exact same percentage for both of the hotels.
But if I go and book a room at the $75/night hotel, show up, and find that they're hitting me with a surprise $50/day "resort fee", then I'm going to feel cheated, and wish that'd I'd booked at the $100/night hotel (assuming that one doesn't have a resort fee, anyway).
The deceptive hotel tried (and succeeded!) in attracting me with a misleadingly low price, when I would have been able to get a better price from a different, non-deceptive hotel. Taxes really have nothing to do with that; people aware of how things are usually done in the US will know that no business would list taxes included unless they were required to, as doing so (when others do not) is essentially a competitive disadvantage.
I do agree with you that a better new law would require businesses to include taxes in their prices. But that's frankly so far outside the norm in the US that I wouldn't be surprised if the bill's author(s) expected it would fail to pass with that kind of language in it, so they limited it to something they actually thought would pass. Baby steps! Incremental improvements are better than nothing.
Also consider that there are many weird people in the US who like having sales taxes added later and itemized out, due to some misguided belief that including taxes up front would let the government raise taxes without anyone knowing. This is related to the unfortunate phenomenon of the IRS not being allowed to essentially do our income taxes for us.