Well truthfully, it has nothing to do with that aspect but what JJ was conveying to Mark Hamill is that for the sequel trilogy, the focus isn’t really on the main trio anymore. Mark Hamill has consistently been on this thing of lamenting that he never got a chance to be with Harrison, Carrie and Billy Dee again which is fine but what one comes to understand is that a story is not going to always cater to your personal desires but rather what a storyteller is looking to do.
But I don’t think that has to account for them not having any scenes together. The issue, I’d argue, is that I think there is no strong story or character reasoning for Luke to not be in the movie and to not have any scenes with Leia and Han.