Also, not all states have the same amount of sales tax and some have no tax at all. It would be impossible to show the prices with sales tax when there are 50 different states with differing sales tax laws.
Heck, the state level sales tax rates are even the easy part. Within states there are counties which often levy additional sales tax, and then cities within the counties do it as well. These county and municipal tax rates are additionally often comprised of a variety of "temporary assessments" which appear and expire seemingly at random. Knowing which rate is charged at a given location devolves into a very difficult problem once you realize there's no centralized address-to-jurisdiction database, nor a centralized jurisdiction-to-tax-rate database.
Current practice is, that the tax of the country, where the company is in-cooperated applies.
With that move, the EU wants, that every country is getting their share, and not like is now, that some countries are getting it. But this added bureaucracy is making it much harder for smaller companies.