I think there is a strong misconception that there is this massive pool of open-source devs twiddling there thumbs just itching to jump in on some project were they can pour time and effort for nothing more than the "good of the community". I don't have any sources for this, but it my strong suspicion that the vast majority of "open source" contributions are actually done by contributors that are compensated either by a company that doesn't mind paying their employees to work on open source projects, or by a foundation behind the open source project. Take Go-lang for example, originally created by Google, then opened up. I am sure there are Google employees that still contribute (on Googles dime) to the project. Why would Google do this? Why not keep the Go just for themselves? Simple, if they open it up and can get other people/companies to use it, then they can make future hires where they don't have to train everyone on a proprietary language.
ReactOS doesn't have a large foundation behind it, and it doesn't make sense for companies to allow their employees to develop and contribute to it on their dime.
Development is skilled labor, especially for an OS. Dev's need to eat, need a home, etc. I don't know a single dev that is itching to give away their skills for 0 compensation. The only time devs really do that is when it is a personal passion project.