It’s very odd that so many haven’t responded but like others, I wonder if there’s a miscommunication here, with people reading it as you just putting out feelers rather than a rock solid invitation.
How long ago did you send the letters? There’s the issue of the postal system, although it’s unlikely to affect 35 people at the same time, unless it’s at your end. One area round here has seen their postie retire but no one has been appointed to replace him so post arrives either once a week or ten days or people go to the sorting office and pick it up themselves.
In truth, I probably wouldn’t go to such an occasion especially if it was going involve travel and hotel costs, unless I was very close to the couple or had known them for many years. I would reply to the invitation, though, I wouldn’t be that rude!