Open core makes sense if you can provide a "light" version of your product that is already useful, and most professional users of your free product will eventually get into a situation where the "pro" version will save them money or will have a feature they dearly need, so they upgrade.

I'm not familiar with Mapbox JS toolkit but maybe the problem was that the free version was too useful already, so only a few of the free users saw the need to upgrade? In that case I can understand why they put it under a commercial license, because providing free support for the project can be a burden.