Why gradually all the fullness of power was in the hands of the majords?

In the VII century. the power of the Frankish kings began to weaken, the concept of “lazy kings” appears in history. Mayordoms, who were something like court ministers, were called upon to replace the king in his absence, but from that time on, more and more power was concentrated in their hands. Now they decide the issues of royal estates, manage the court service, and concentrate military, judicial and administrative power in their hands. This situation, when the king was only nominal, and the country was ruled by a mayord, remained until 751, when the last king of the Merovingian family was imprisoned in a monastery by Mayord Pepin the Short.