It is called 'seiryoku' because there was no center of power as with the military government and Imperial Court, but each religious institution acted independently and even in one institution there were many counteracting groups.
武家政権や朝廷のように権力中枢があったわけではなく、各寺社が独立して行動し、また一寺社内でもさまざまな集団がせめぎ合っていたため、「勢力」と呼ぶ。