It hurts when a close friend turns down a wedding invitation, especially when there’s no explanation.
Having been to several large weddings last year, I can think of a host of reasons the close friend declined the invitation. Here are just a few: the cost, the perceived need to buy a new outfit, the date and location, jealousy or other negative emotions, social anxiety, being photographed, realizing your age, feeling sad about the passing of time, feeling ignored or invisible, regretting one's own choices, disliking your spouse, recently quitting alcohol, etc. Your response was good -- the only choice is to ask the person why they are not coming. If they don't answer, I'd say the friendship is likely affected forever.