Poloniex has moved to the Seychelles, which is the “Land of the Free”. If you don’t desire the protections offered by the Seychelles, please use Coinbase. What’s wonderful about the crypto industry is you can voluntarily choose the exchange you like!

Sorry for being unclear- I meant why (i.e. what was the reason for the decision, not where did they move to) did Poloniex move from the United States to the Seychelles, and stop allowing users from the United States?

due to US laws. Same reason Bitmex is moving to requiring KYC and Binance created a US only site - to avoid getting sued/shut down.