For a lot of reasons the SIM leaderboard is not particularly accurate, and is meant simply as a good-fun exercise. It's not accurate because (for example) you can jazz up your results by trading with more capital than the 100,000 in your SIM account would normally allow.
So, when results get too ridiculous, we just stop showing them. That way, we avoid regulatory and compliance issues. If you'd like, you can reset your SIM results so that you can start tracking from zero again, and then you will begin to appear again on the leaderboard.