Because Mississippi didn't have Brexit /s
All jokes apart I don't see any real evidence that Mississippi is richer than the UK. Their median household disposable income is pretty damn close ($49k vs $54k). The UK has a better social and health safety net (where would you rather be unemployed or disabled?).
Mississippi has much lower life expectancy than the UK (69 vs 79 for men). Crudely speaking a Briton makes $5k less/year but lives 10 years longer - a trade almost anyone would make in a heartbeat.
I won't bother digging into other quality of life indicators vs economic indicators but I expect Mississippi will be mostly equal or worse.
On average Mississippi may have higher dollar numbers on a spreadsheet. That doesn't mean squat for the ordinary person on the street. If anything this is an argument against trickle-down economics/the Laffer curve.