The issue with the oceanic flights is that there are no receiving stations for the ADS-B messages, as these stations are ground based. Therefore, there is no way to receive the position reports to feed into the flight data network.

I think that's because there are no data for that region - the FAQ mentions that the data is crowdsourced from folks with certain receivers. Although, I can't imagine it'd be hard to simulate the paths based on data from both coasts.