Why depend on popularity to make it open source? Shouldn't it be the other way?
He light actually get more contributors by making it open source right now instead of waiting for something that might never come.
Random take, am not trying to defend it, just thinking of reasons - maybe he doesn't want people to fork it and risk that someone would market it better than him, and he would end up only as a footnote in sources list? So he wants to get known as The author, before making it open source?