Acharya Prashant explains that the primary reason people fail to complete their to-do lists is that the tasks listed are often worthless or lack personal significance. When a list is filled with items that one does not love or value, the individual relies on willpower to push through, which is often insufficient. This leads to a cycle of doing just enough to survive without any real commitment or passion. He emphasizes that the problem is not a lack of motivation or willpower, but rather an absence of understanding and love for the work being performed.