The prevailing thought is that since most of the rest of the world is still using the easy-to-give oral (live virus) type, then that is how it is getting re-introduced. The U.S. stopped giving that and went with the injection of inactivated after ~1997 I think.
I expect it has been circulating for awhile from overseas travelers, but no one was looking for it until that one unvaccinated person experienced paralysis from it in NY. And that is when they started testing the waste water for it (they were already doing regular COVID-19 waste treatment testing) and sure enough, the same genetic profile of the Polio virus that individual had, matched what was in the sewage sheds. Similar has happened in California with monkeypox wastewater testing.
I hope the booster I had 30 years ago still works (needed it to go to Egypt and it was on the traveler immunizaion list along with yellow fever and typhoid)! 
I know my mother (who I went with on that trip) was like WTF? although she would NEVER say those words 
) "Why do I need this again?". She was a teen in the '40s when Polio was rampaging around the U.S. and got the vaccine back when it became available in the '50s when she was in her 20s. When we got the booster, we had the oral one (they were just starting to switch to the injection around that time but apparently all the doc had was the oral one).
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