The HC are planning the wedding that they want in a location that they like/can afford. They probably selected Thursday, as others have suggested, due to the lower costs involved. I am sure that the cabins with bathrooms were booked by members of the family/bridal party. That leaves the other cabins and the b&b's on the island for other guests who are attending.
I am sure when they booked this site and choose to do it on a Thursday, they realized that not everyone would be able to attend. Choices have consequences, and when we make choices we have to live with the consequences. So, HC can send out their std's and then the invitations, but cannot get mad when some people that they may have liked to attend rsvp "no". They may be thinking 'this is the day we have selected and we have checked with our bridal party and close family and it works for greatest majority of family, we can have it at the place we want and save money; so this is what we want and those who can attend will attend, and those who can't will be missed."
No HC figures that everyone they invite will be able to attend. Invited guests have a life of their own and the date that a HC has selected may not work for all of their guests. HC should be happy about the guests that are able to attend. So, I doubt that this was done deliberately to offend or hurt you (or other guests who live far away) or to send a message that you aren't important. They could have easily picked another date==on a weekend with lots of nearby accommodations--that wouldn't have worked for you either.
They may not realize how inconvenient it would be for guests traveling across country by air to bring sheets, blankets, pillows, towels, dishes, etc. Maybe they think those flying in will buy and then donate the items, maybe they don't have a clue, maybe they just don't care, maybe they have a plan for supplying these--have you asked about that?
If you want to attend, check out the b&b's on your own. I would not wait for the couple to assist you on this.
If the b&b's don't work for you, you need to reassess whether you want to lug all of that stuff along with you or spend the money to buy it there and then perhaps donate it to an organization. Do you want to traipse outside at night, maybe in the rain, to use the bathrooms? Would you get a good night's sleep? Can you afford to miss work during the middle of the week? If this is more than you want to do in relation to the cabins or can't take the days off from work, then decline the invitation and move on--send a nice card/gift and wish them well.