Price and convenience. Can you get a non-stop into STT? The fewer moving parts you have, the less chance for a FUBAR that messes you out of a day or so of vacation. From many parts of the country, it is a lot cheaper to fly into STT, even when you add on the ferry. However, if you have to have a connection just to get to STT and flying into EIS isn't that much more, that's the way I personally would do it. Your mileage may vary.