I’d hazard a guess it’s because the majority of drones currently on the market don’t use the Galileo Constellation of Global Positioning Satellites, but rather only the US GPS and the Russian GLONASS.
I don’t know the drone manufacturers reasoning behind it as most GPS receivers these days are capable of using more than two different constellations. Using an opensource firmware hack I was able to configure my Spark to use at least three of the available constellations.
Regards
Nidge