Two major reasons: Capacitive losses. They're worse in water (dielectric), but they're there in air lines as well. Compatibility and isolation of grids. If gr

Two major reasons: Capacitive losses. They're worse in water (dielectric), but they're there in air lines as well. Compatibility and isolation of grids. If grid A is 35 degrees from grid B it's very hard to couple them. Also, cascading failures tend not cross DC boundaries.