Have you point-blank asked them why they won't visit?
We are very close with my IL's. We see them A LOT. Right now about once every 8 weeks or so, sometimes more often, sometimes as long as 3 months between visits. They live in FL and we are in OK. They usually come to us because it's easier that way as we are 5 people. We went to them at Thanksgiving last year and haven't thought about Thanksgiving this year yet.
When we lived in Italy for 2 years (2008-2010), they visited us in the summer and at Christmas each year (they are teachers). They are on the verge of retirement and a very active 65 and 67.
My family, on the other hand, we are not as close to. I saw them when we were home for Thanksgiving and will see them next in September. So I see them about once a year. Wish that was more often but it is what it is.
M, mommy to A 4/05, E 1/07, and L 12/10