Answer
The king resisted because it was giving Pope Gregory VII the authority to extend power over all christian world including rulers.
Step-by-step explanation
King Henry IV fought over the lay investiture by appointing high ranking clerics/bishops as vassals for administrative purposes so that he could maintain his power as King.The pope issued a decree forbidding bishops from receiving investiture from lay leaders like the King aiming at establishing a system where the Pope has more power and authority than any leader.The king ignored the decree because he saw the need of putting more administrative individuals in his trust to facilitate his ruling.