Why God allowed it? We do not know. Did God plan for the fall of Adam? No. God foresaw the fall and God permitted the creation to go through knowing that Adam would fall causing death to come upon the whole human race.
You may disagree with this statement but it is what the scriptures teach concerning our fallen world.
Many within Christendom attempt to fix the dilemma here insisting that man is responsible for rejecting God and thus becomes damned. The Bible is very clear that all men are already condemned and headed to hell from the womb. That is why sometimes there is infant deaths because even before the child commits their first sin, death is for all.
As a Christian here is the proper view concerning the God of Abraham Isaac and Jacob.
God foresaw that the fall would take place and permitted the creation to move forward because He planned to reach into humanity and save some from the masses while passing over others. There was no merit involved in the election of those who will come to faith. It was purely the outworking of the love of God and the grace of God.
Blessin’s