Because those are two unrelated taxes.

Both the Federal government and State government can tax you.

States don't control federal taxation. The state choosing not to levy a sales tax doesn't mean no one else is allowed to.

Well, tariffs and other such import fees aren't a tax on the sale of an item. They're a tax on shipping a package in from overseas. Not actually the same thing.

Same reason people who live in states without an income tax still have to pay the IRS.

Different taxing entities all want their money.

Because the house always wins.

The simplest answer to this why question is that them's the rules.

A tariff is also different from a sales tax. One doesn't preclude the other. I'm sure there are places in the US where you would end up paying both. The tariffs are a federal instrument of regulating cross border trade. Sales tax or the lack thereof is a state decision.

Because the state and federal governments both have the ability to levy taxes and the import tarrif is a federal one. Also, even buying retail in Delaware, that item has already been subject to an import tarrif, and the state just declines to charge a sales tax on top of that.

Because import duties and sales taxes are different things.