The reason that is is because God chose to have it that way!
He wanted creatures to freely choose for Him, which meant that the choice had to be a real one, not directed by Him.
God could have made us all loving Him and there would have been no Fall.
God was willing to endure the rejection of some, to have the free love of others.
That is the very nature of love, a desire for a true response.
Calvinism tends to think that God is some sought of inpersonal force, indifferent to the response of man, but on the contrary, God is always striving with man, until man's own rejection is so stubborn that man must now face God's wrath and not His love. (Acts.7:51)
There you go again, projecting your false understanding of Calvinism onto a faulty position. God is by no means seen as impersonal or indifferent. On the contrary, He is extremely engaged and involved. By the very presence of your doctrine of prevenient grace you are in a sense invalidating your statement that "He wanted creatures to freely choose for Him, which meant that the choice had to be a real one, not directed by Him." If that were true, He never would have interfered and we'd all be lost.