They fall because they choose to.
Life is about choices and God gives us choices.
We can't blame the devil for something that WE did- the devil can only offer what you want and desire.
It's up to us to accept.
we fall because we are sinners that is just the case again 1john 1:9 . the bible tells we are born in sin we dont choose to sin we are sinners and we must work on not sinning . The reason why the word of God has people who have fallen in it is for us to learn from them and they were still used by God despite there human flaws . That is the whole key to living . Paul says that of sinners he is chief so did he choose to sin at that time are was he a sinner who got saved but still had the OSN