Judging by the renderings of the proposed Belle Isle, it looks like he intends to have significant infrastructure there including what are clearly some very large skyscrapers.  It could very well be that the islands you posted do not have the proper soil/foundation to support such large infrastructures.  They're also only 117 to 250 acres in size (a lot of which looks unusuable, Bird Cay has a large body of water within the island) while Belle is 928 acres in size and located next to a major city making import/export a lot easier.

Belle Isle is probably a pipe dream but the guy has a lot of money and is looking to establish among other things, a new currency.  Why re-invent the wheel when he can just use Bitcoin.