The question accepts that every soul is subject to original sin, it then goes on to ask why. One can peal back one layer and say that every soul is subject to original sin because Adam and Eve committed a personal sin, but that does not tell why they did.
Can humans obey God's law without God's help/grace?
Could God have given Adam the grace to obey?
Why did God withhold from Adam the grace to obey?
I think these 3 questions are all part of the why which is being asked.
Ans 1. In Job the Devil obeys God but not God's law of love. To obey God's law we need the power of God that raised Christ from the tomb to work in us.
Ans 2. If God had had a different plan for creation then things would have been different. But given the plan He had it was not His intention that Adam would precede Jesus in obedience.
Ans 3. God alone is holy and therefore His creation, which is not Him, is not holy. God is holy in demonstrating this, which He does by giving a command that Adam will fail at. Adam's failure becomes Christ's opportunity.