ADS-B has the same problem -- it's hard to collect it over the oceans.
Satellites do pick up the signal, but the detection area is pretty wide, so sometimes it's just too many signals coming in all at once, overwhelming the satellite. ADS-B is also pretty chatty, so the data sizes are significant (compared, to, say AIS, which is a similar system for marine vessels).
It's gradually becoming better, satellites having more focused antennas able to gather signals only from unpopulated areas, and more computing power to filter noise out.
Satellites do pick up the signal, but the detection area is pretty wide, so sometimes it's just too many signals coming in all at once, overwhelming the satellite. ADS-B is also pretty chatty, so the data sizes are significant (compared, to, say AIS, which is a similar system for marine vessels).
It's gradually becoming better, satellites having more focused antennas able to gather signals only from unpopulated areas, and more computing power to filter noise out.