It really hard to stay motivated. Sometimes your reason for not finishing a project is a lack of knowledge or skills. You can find lots of material online to help with this however, every game is different, and the solutions vary a lot between them. You may need to level up your skills before you can finish something. This can be demotivating especially if you don't grasp a concept quickly you may have to do it several times to fully understand. Persistence is a desirable trait to have when making games. Making Games is hard.
We hope to provide an incentive for developers to finish stuff as well as get things tested with real players. We also encourage testers to roast the creators hard and provide honest feedback. I guess it comes down to who is making it, how much you believe in the project, and the feedback you get. These are things that can motivate you to finish a project or abandon it.