I suspect that some of those places host strategically important communications facilities. Perhaps spy satellite downlink facilities, perhaps GNSS ground-segme

I suspect that some of those places host strategically important communications facilities. Perhaps spy satellite downlink facilities, perhaps GNSS ground-segment base stations, perhaps earth stations for other communications satellites. Oddly, those two specific islands don't. Saint Helena is specifically courting such business, but I don't see any indication that they've attracted anyway. But several other small islands are definitely fibered-up on account of their satellite facilities.