It's a LOT of work to open-source something which has been evolving at a huge pace and is as complex as the Realm core.

Even just announcing the work of the Xamarin C# team that I'm on got some complaints from the public about this representing a "loss of focus". So yes, you could say there was a "business reason" of protecting the core team whilst they got to 1.0.

If you look at the C++ portions included in most of the Realm open source you can see we have moved code gradually into the open source codebase. There's not yet an official C++ API either for the same reason - it would lock in too many decisions and tie up the team. (disclaimer - these are opinions and observations from a remote contractor, not Realm management!)

I don't believe that complexity or size of code really affects the difficulty of open sourcing a project. Its more likely the culture and daily development practices that are difficult to change, but probably not as difficult as the business strategy we're all wondering about.