Neither one of the sides were fruitful in getting through the resistance of the other and bringing about a stalemate. Trench fighting developed due the disappointment of the Schliffen Plan. It was another arrangement of fighting which was an immediate reason for the stalemate, and this is the reason. It was an arrangement of resistance not offense.
The best and correct answer would be It was in an area of the world that emperors did not want to conquer. This is a strong country with brave emperors which the other countries do not want to deal with.
