I think many people don't realize how inexpensive they can truly make the trip...and then I think others use the cost as an excuse not to come, because they're nervous or unsure how to go about planning what they see as a HUGE trip in terms of distance, a new culture, etc. etc. etc.
I have a few friends who have expressed interest in visiting, as well as a few college-aged cousins, but I don't expect to see my family over anytime soon, if ever. My mom confirmed this when she said she couldn't attend the wedding I was planning to have in England. I was really upset for a while, but I thought about it more and realized that I hadn't truly expected to get many people over for the wedding anyways (and eventually I realized I didn't want to spend money on a wedding, hence the decision to elope, but that's another story...). Everyone has their own reasons why the trip seems insurmountable to them. For my mom, she views it as more expensive than it probably is and is also terrified of flying. For some, the thought of leaving the country truly scares them, or doesn't interest them at all, or seems like too much of a hassle to plan... and sometimes life just gets in the way! I think back to when I had several friends living abroad, and though I did manage one visit to Germany, I had visions of visiting them another time or two that just never happened due to needing to organize a move, using all of my vacation to go back home and visit family, being too busy with work, and on and on and on.