It came out of an ARRL field day event called NPOTA ("National Parks on the Air"). People loved that event and so the POTA organization was created to continue

It came out of an ARRL field day event called NPOTA ("National Parks on the Air"). People loved that event and so the POTA organization was created to continue it, extending the valid parks to state-managed parks (in the US, other countries have their own park inclusion criteria).