Free will exists because Krsna allows it, just so the jiva-souls can make their own voluntary contributions. Only by Krsna’s sanction do we have the free will to do this.

Free will, or the ability to choose and contribute in your own unique way, is eternally part and parcel of the jiva-soul's (marginal living entities) natural constitution and ALWAYS exists without beginning or end in the Spiritual Planets. 

However, free will in its full potential, is ONLY fully experienced in Vaikuntha and Goloka Vrindavan, which is the jiva-soul's original perpetual home.

The material creation exists for those jiva-souls who choose to fall down from the spiritual world's of Vaikuntha and Goloka Vrindavana.  

However, the material worlds are NOT the real natural home of the jiva-souls, and the jiva-soul's full freedom is never experienced in the material creation due to being covered by material energy, therefore, their natural home is NEVER in the material worlds.

Devotee - "Srila Prabhupada why did Krsna give us free will if He knew we may rebel and miss use it and choose to enter the material creation?"

Srila Prabhupada - "If you have no free will, then you are a stone. The stone has no free will. You want to be stone? Then you must have free will! But don't misuse your free will. But don't try to become stone either. That is not life". (August 5, 1976, New Mayapur France)

Srila Prabhupada - ''As soon as we try, "Oh, this material world is very nice," "Yes," Kṛṣṇa says, "yes, you can go." Otherwise what is the meaning of free will? Every living entity has got a little free will. And Kṛṣṇa is so kind, He gives him opportunity, "All right, you enjoy like this." It is free will''. (Morning Walk At Cheviot Hills Golf Course, May 13, 1973, Los Angeles)

Devotee – "In Srimad Bhagavatam it says that Krsna did not want us to come to this material world. If Krsna did not want us to come, why are we here?” Why doesn’t He save me from thinking like that?”

Srila Prabhupada - “That means you lose your independence. That is force, in Bengali it is said, ‘If you catch one girl or boy, ‘You love me, you love me, you love me.’ ” Is it love? “You love me, otherwise I will kill you!’ Is that love? So Krsna does not want to become a lover like that, on the point of revolver. ‘You love me, otherwise I shall kill you!’ That is not love; that is threatening. Love is reciprocal, voluntary, good exchange of feeling. Then there is love. Not by force; that is rape. Why one is called lover, another is called rape?” (July 8, 1976 in Washington, D.C).

As Srila Prabhupada explains below, the relationship between Krsna and the individual jiva-soul is never a one way relationship that denies personal contributions and choices, no, love is ALWAYS based on the reciprocation between "two" because love can only exist when there are "two" engaging in loving exchanges as Prabhupada explains-

Srila Prabhupāda – ''Unless there are two persons, where is the question of love? Love means two persons, then there is exchange, then there is love. I must deal with you open-hearted, you must deal with me open-hearted, then there is love. So if you want to love Kṛṣṇa, God, then these things must be there". (August 9, 1976, Tehran)

Srila Prabhupada – “So everyone can know that independence means one can use it properly, or one can misuse it. That is independence. If you make it one way only, that is not independence, that is force.” (Los Angeles, June 23, 1975)

Srila Prabhupada - “So, unless there is a possibility of misusing our free will, there is no question of freedom.” (Dialectical Spiritualism, Critique of Descartes)

Ultimately each individual soul is a unique person who can choose for themselves to voluntarily serve Krsna the Supreme Personality of Godhead and cause of all causes, or ignore Him. 

As Prabhupada explains, Krsna never forces you to surrender to Him, never forces you to love Him, that must be voluntary.

Srila Prabhupada – "Krsna NEVER forces you to love Him, that MUST be voluntary. (Melbourne June 25, 1974)

Srila Prabhupada – "Love is reciprocal, voluntary, good exchange of feeling. Then there is love. Not by force, no, Krsna does not want to become a lover like that." (July 8, 1976)

In the following conversation Srila Prabhupada is explaining to us that Krsna is ALWAYS in control because He is the Supreme Personality of Godhead and cause of ALL causes.

This includes allowing the jiva-souls to have their independence and free will explained here-

Syamasundara dasa - ''Does the individual soul have a little independence to choose?"

Srila Prabhupada - ''No, Bhagavad-Gita states that when the individual soul wants to act, Krsna gives the orders. Man proposes, and God disposes". (Dialectical Spiritualism: A Vedic View).

What Srila Prabhupada is saying here, is the individual soul cannot have their little independence or free will without the sanction of Krsna first. This is because Krsna is the cause of ALL causes, it is His will alone that allows the jiva-souls to have their freedom.

Syamasundara dasa - ''So we have no free will?"

Srila Prabhupada - ''Not without the sanction of Krsna. Without Him, we cannot do anything. Therefore He is the ultimate cause''. (Dialectical Spiritualism: A Vedic View).

So free will exists as part of the constitutional make up of every marginal living entity (jiva soul) because Krsna allows it, only by His sanction do we have free will.

This is an obvious and only conclusion because without free will, there can NEVER be genuine loving personal contributions, exchanges and reciprocation with Krsna, we would simply be like dead stone as Prabhupada has explained.*.*.