Because as you rightly pointed out it is not God's will to save everyone. It is His desire to save all but it is His will that each should choose to love Him from our own free will. This is what His grace does in that it controls the desires of the flesh when the truth is presented so one can freely choose for themself without compulsion or manipulation just as Adam and Christ chose to accept/reject of their own will. Adam condemned us, not God. God set us free by removing the barrier that prevented us being able to come and worship with the Cross. He then supplies the grace needed for us to see the truth and respond freely from our own soul.
But it did have the positive effect God willed by freeing the person who is presented with the Gospel. The problem is you think it is meant to save the person in order for them to believe so the Gospel can save them. You're theology is illogical. How many times do you want God to save you from death?
I dare say your misunderstanding of what I think would lead you to think I think God's grace is not a miraculous power but I know it is. It raises men from death. You simply want it to do what you want it to do and not what God want's His grace to do.
His grace to raise a person to life comes after the Gospel is accepted, that is the difference between you and me. You think a man has to be saved from death and transformed before he can believe and be saved. I think a man can be made free enough to accept or reject the Gospel of his own volition. God does the rest and He always initiates.
You do that. Hopefully the Lord will be gracious to you so you can fully comprehend what the Cross did and didn't achieve.