Hi, I'd like to help you with your question.
It sounds like you are being more than fair in trying to help them visit. Offering to pay expenses and inviting them often is definitely generous and should make it a lot easier for them to visit. Since they will not, you are left to wonder what the issue could be.
There could a variety of reasons they won't visit. For instance:
One, they really do feel you live too far and think the kids would not tolerate the traveling well at their ages. Some parents are protective of their children and do not want to impose situations on the kids that they feel may cause them distress.
Two, they have an issue with you, your husband or both. Sometimes when children get married they change their relationships with their parents either because of a different outlook or their spouse has issues. If your daughter in law has issues, she may make it hard for your son to take the family to visit you. You can talk with your son to see if there is some personal issue they have with coming to visit.
Three, your son's wife feels uncomfortable. She may prefer the company of her own family and feel odd or unsure of how to relate to you and your husband. Some people are very shy or have personality issues that interfere with their ability to get along with others.
Four, your son or his wife fear traveling. This is a long shot, but some people feel very uncomfortable at the thought of traveling far from their home. And people that do have this fear are usually embarrassed by it and try to keep it private.
One of the best ways to find out what is going on is to try to talk with your son about it. If you can, do this face to face. If not, find a time you can talk with him on the phone or Skype privately so he feels more free to talk. Be blunt and ask. Tell him you are concerned and just want to understand why this is happening. Try to stay neutral and non accusing so he feels more free to express himself.
Even if you try everything, you may not be able to convince your son and his wife to visit you. But even if you do have to go to them, be sure your grandchildren know you love them and think of them often. Show them pictures or DVDS you made of your home and things you do in your life that are important to you. That way, they at least can develop some connection to your life at home.
I hope this has helped you,