I'm really surprised that so many posts suggest that you keep contacting them, whether to ask why, or to ask them out to lunch or just send a card or just call or...
Ugh, please don't. They've already made it very clear that they want you to leave them alone. So I think continuing to contact them anyway would be quite dense and pushy.
Very few friendships last a lifetime. We probably all know what it's like to feel like a friendship has run its course and want to move on. When people want to move on, they're past wanting to discuss it. They rarely say why.
And what would they say anyway? Sorry, but your constant prattling on about your health issues/dog/grandchildren just bores me out of my mind. Sorry, but if I have to listen to your husband brag about his money one more time, I'm afraid I'll choke the life out of him. Oh, and also....
Sorry they've moved on, though. It's not a nice feeling. 