I've been on both ends of this. I don't like doing it to others, but from when I have, it's because I was either ambivalent about the person in the first place, or I had other things going on that were distracting me from getting more involved.
It all boils down to that harsh saying, "s/he's not that into you." Which is always an ouch to hear.
I think a lot of relationships have more to do with timing than anything else.