I'm guilty of being one of these ghosts. Perhaps it's bad conduct, but when I've decided to move on from the solution that the sales person was providing I generally don't get back to them. As such, I'll list some reasons as to why below to maybe make up for bad karma build-up...
The reasoning behind it stems from one of three things; I've found a solution to my problem so I don't need your service/product any longer, the problem I was having has been superseded by something else so I've shelved it for the foreseeable future, or I've discussed your product/service with the upper-management and they've got back to me with a big red 'denied'.
I suppose the solutions to the above would be as follows:
1. Ask whether the buyer is considering other options in the same market of the service/product you're providing. As the buyer, I'll be messaging up to 20 different sales-people at any one time to see what they can offer me compared to their counterparts. There's nothing more frustrating for me than a sales-person who beats around the bush - so even if their offer might be better, it's not worth my time to organise meetings, discuss price, email back and forth, and do other busy-work with them when I have another sales-person that is transparent with what they are offering from day 1. Succinctly put, find out who you're competing against quickly and close the deal before they can offer something better.
2. Again, this is in the realm of closing the deal quickly. I often get jobs that crop-up mid way through negotiations, that supersede said negotiations. There's not much you as a sales-person can do in this case other than close quickly, or be persistent with contact so when the job does come back onto my radar I've still got your contact at the back of my mind.
3. Discuss whether the buyer is at the whims of a 'upper-management sign-off'. If they are, you can discuss with them as to how to make it easier for them to get that sign-off. I recently had to buy a new CAD package. There ended up being some contention between myself and the uppers due to integration with our current systems, but one of the sales-people and I talked through the issues I was having with management and rectified it, going as far as writing some new code to help with the integration before we even started to talk about price. The other sales-persons basically said "Hope you get it sorted. Lets us know how it goes" and that was that. Obviously we went with the prior package, almost entirely due to the service.
At the end of the day, there are 101 other reasons, but those are the top 1, 2 and 3 I can think of currently. I hope that helps somewhat!