Things do happen. Real life can be hell on me for a period of time and keep me from developing. I've got such a time coming up soon.
To keep it short and sweet, developers are people. Sometimes they do get sick. Sometimes shit does happen. There are also times a developer loses their drive or interest. It can take a long, long time to make a complete product, especially for a single individual.
Sometimes they get scared - depending on what they're making. Yes, sometimes, it's a money grab. A few dollars to someone in a poor part of the world can mean a whole lot. It's relatively easy to make something and put it out there.