Historically cost. Building a non-synchronous grid is much harder because you need equipment that can correct for phase differences in your grid. This equipment

Historically cost. Building a non-synchronous grid is much harder because you need equipment that can correct for phase differences in your grid. This equipment used to be very large and very expensive, Mercury-arc valves are some of equipment that used to be used (they provide AC to DC conversion) [1]. But modern power electronics are so cheap and efficient that High Voltage DC transmission lines are becoming increasingly common, driven by the fact the High Voltage DC is more energy efficient that High Voltage AC (you can push more power down the same cable, with less energy lost to resistive loads because DC doesn't suffer from the skin effect [2]). With more High Voltage DC transmissions lines it becomes easier to run non-synchronous grids, because the DC transmissions acts as natural breaks between synchronous grids. Running a non-synchronous grid tends to be a little easier because you no longer need to worry about strange phase variances caused by multiple transmission lines of different lengths injecting phase delays into your network. [1]: https://en.wikipedia.org/wiki/Mercury-arc_valve https://en.wikipedia.org/wiki/Mercury-arc_valve [2]: https://en.wikipedia.org/wiki/Skin_effect https://en.wikipedia.org/wiki/Skin_effect