The reef is the whole story. It runs parallel to the north shore of Providenciales, absorbing the swell before it reaches land, which leaves Grace Bay calm even on windy days. Shallow water over white sand is what produces the banded turquoise that shows up in every photograph of the island.
The islands are low and dry — scrub and salt flats rather than rainforest — so the light is hard and clean, closer to Aruba than to St Lucia. The US dollar is the local currency and flights from the US East Coast are short, which removes a lot of the friction that makes other Caribbean destinations complicated for American guests.