Why? Compared to most of the rest of the Caribbean, the North Coast is relatively inexpensive. It's also reasonably safe, has lots of good beaches and many good markets. There's one solid clinic, and thanks to the improved highways, most of the North Coast is less than three hours from HOMS in Santiago and the SD hospitals as well as three international airports. The fact that you're only a 90 minute flight from American soil ain't bad either.
It's good good weather, decent food, and all the, ahem, "companionship" a person could want, all for the price of social security and a decent supplemental pension/annuity/savings fund.
Personally, I'm shooting for San Diego or perhaps Hawaii (I love being in America too darn much, curse me), but there are far worse places to pitch your tent than DR, especially if you've got a solid income stream.