This is because, by doing so, succession of the throne by a male on the male side can be maintained, even when the female member of the Imperial family gave birth to a son and he became the Emperor, and restoration of former Miyake (house of an imperial prince) is thought to proceed smoothly without opposition of the public.
こうすることで、女性皇族が男子を産み、その子が天皇になったとしても、男系男子による皇位継承を維持できるし、旧宮家復活も国民の抵抗なく円滑に進むと考えられるからである。