Lack of ground based ADSb tracking stations.
ADSb coverage capture shows the same lack of tracked planes over other oceans, Canada, Russia, N Asia, Africa, & S America.
pf
No, it is just the nature of the beast.
The system we monitor needs ground feeder stations around the world to set up ground feeds.
Just like a ordinary radar it doesn't see over the horizon rather like a cell phone losing signal when out of range of a tower.
Similar ship trackers use satellites rather than ground based trackers so don't suffer the same problem when they move away from land.