I would assume it has to do with fuel prices. The BVI's are a stones thow away, St. Croix is much farther. Take a look at Google Earth and I think that you would understand. I'm not sure, but I thought that they didn't offer ferry service anymore and that you had to rely on a seaplane. I'd like to here more on this.

does this include a vehicle? from stt to stx?

There is ferry service between STT and STX 4 days a week, Friday, Saturday, Sunday and Monday( I think those are the days). Round trip twice a day, once in the morning and again in the afternoon. STX is about 45 miles from STT, much further away than STT from STJ or from STT to any of the BVIs.

STT and STX are part of two different mountain ranges. The trench between the 2 islands is very deep and the crossing, which takes about 90 minutes, can be rough. This is not a car ferry, people only. There is no car ferry. Shipping a car between the 2 islands is not for the day tripper, or even really the week tripper.

All that being said, the ferry service is reliable (unless the seas are too high for the crossing) and the cost seems reasonable compared to the seaplane or cape air.

I've heard the people ferry to STX referred to as the "Vomit Comet". ....not a very reassuring nickname.