I asked Ting Thing the same question in a private message on the old forums. He said "I stopped working on Mario Builder for several reasons, most significantly that it was too time consuming for me, and I built it up to a place that I was proud with. If I could have marketed the program like I do with Platform Builder, I probably would have continued developing it. But since it used so much copy-written material belonging to Nintendo, I didn't feel it was morally right and I definitely didn't want to get into a legal battle with them. Another reason I stopped was because I was concerned that Mario Builder would eventually be shut down by Nintendo, and I didn't want to invest so much time into a project that would eventually have to stop. I turned out to be right. About a year ago, Nintendo wrote to GameJolt and asked them to remove a large number of fan games which contained material belonging to them. Mario Builder was on their list. I could still work on it and upload it elsewhere, but at that point I would be knowingly going against the will of Nintendo." That's an exact copy of what he sent to me.
