Feedback is voluntary and some people don't see it as a necessary thing. I'm not that worried about feedback now as I have enough, but I can fully understand why it is still important to you. I'm not sure sending buyers a reminder message is a good idea because that can upset some buyers and they could retaliate by trashing your feedback. Some will suggest leaving it for the buyers first. That may or may not work. Some buyers think, well I've got MY feedback, stuff the seller!

These days I don't leave it until the buyer has left it for me. I'd say around 50-60% of my buyers leave it for me. When I used to leave it first, I rarely got it back, which is why I stopped leaving it first. 

I don't know if there are any real answers as to why buyers don't want to leave it. Think of it as being 100% positive feedback for those that don't leave it. It means they have got their item and are happy with it, so they won't open a dispute or trash your feedback. It doesn't help get your numbers up though!!