Anna - I understand your point. It's frustrating when people simply won't commit, and oddly enough, this is one of my MIL's biggest "gripes" about me...LOL. She's complained that we don't make the 7 hour trip up there enough, and she's got a point. We really don't go that often, a just few times a year (even less now that we have a baby, and the expenses that come along with her). Of course we go even less now that we don't have any contact with her. We try to maintain contact with the other members of the family by phone, email, facebook, etc...but we don't make the trip much anymore.
"Lack of commitment" could happen for a number of reasons. It really could be that they are waiting on a better offer, or it could be that they have "tentative" plans already, and need to see if those plans come to fruition, OR....
speaking for myself and my family (DH and child) here....
Sometimes I just need a break. It's not that I don't want to commit to an event...it's that I already have so many other plans
around that date, that I just need some "home" time. No family visiting, not going to anyone's house for dinner, no shopping trips, no in-laws coming for the weekend, no traveling, no entertaining....I honestly don't want to commit to anyone or anything other than my husband, my baby, my pajamas, and my sofa.

Just my two cents...