- the package dependencies, packages get often deprecated on npm and in rare occasion contained malware, Qt contains most things you need and a web view if you need to show html or run JS for some reason
- GCompris is an old project and it had one big rewrite to Qt in latest years, using Web tech they would be forced to keep up with the web frameworks, imagine you will have to rebuild your current web project in 10 years from now and continue working on it
- I think a port to web tech would have negative effect on the battery life of the devices
On the upsides you could have the web port running in a browser, though having something that works without internet is a big plus, there must be similar projects or collection of different web projects somewhere but others mentioned Windows and mobile devs will think how to make money in the app stores maybe with some ads.