Because they have not known the Father - See on John 15:25 (note). Ignorance of the benevolence of God, and of the philanthropy of Christ, is the grand fountain whence all religious persecution and intolerance proceed.

because.

John 8:19,55 Then said they to him, Where is your Father? Jesus answered, You neither know me, nor my Father: if you had known me...

John 15:21,23 But all these things will they do to you for my name's sake, because they know not him that sent me...

John 17:3,25 And this is life eternal, that they might know you the only true God, and Jesus Christ, whom you have sent...

Luke 10:22 All things are delivered to me of my Father: and no man knows who the Son is, but the Father; and who the Father is, but the Son...

1 Corinthians 2:8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.

2 Corinthians 4:3-6 But if our gospel be hid, it is hid to them that are lost...

2 Thessalonians 1:8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:

2 Thessalonians 2:10-12 And with all delusion of unrighteousness in them that perish; because they received not the love of the truth...

1 Timothy 1:13 Who was before a blasphemer, and a persecutor, and injurious: but I obtained mercy, because I did it ignorantly in unbelief.

1 John 3:1 Behold, what manner of love the Father has bestowed on us, that we should be called the sons of God: therefore the world knows us not...

1 John 4:8 He that loves not knows not God; for God is love.

1 John 5:20 And we know that the Son of God is come, and has given us an understanding, that we may know him that is true...

May 14 Morning
The fellowship of His sufferings.--PHI. 3:10. It is enough for the disciple that he be as his master, and the servant as his lord. He is despised and rejected of men; a man of sorrows, and acquainted with grief; and we hid as it were our faces from him; he was despised, and we esteemed him not.--In the world ye shall have tribulation.--Because ye are not of the world, but I have chosen you out of the world, therefore the world hateth you. I looked for some to take pity, but there was none.--At my …
Anonymous—Daily Light on the Daily Path

Peace and victory
'These things I have spoken unto you, that in Me ye might have peace. In the world ye shall have tribulation; but be of good cheer; I have overcome the world.'--JOHN xvi. 33. So end these wonderful discourses, and so ends our Lord's teaching before His passion. He gathers up in one mighty word the total intention of these sweet and deep sayings which we have so long been pondering together. He sketches in broad outline the continual characteristics of the disciples' life, and closes all with the …
Alexander Maclaren—Expositions of Holy Scripture: St. John Chaps. XV to XXI

Why Christ Speaks
'These things have I spoken unto you, that ye should not be offended. They shall put you out of the synagogues: yea, the time cometh, that whosoever killeth you will think that he doeth God service. And these things will they do unto you, because they have not known the Father, nor Me. But these things have I told you, that, when the time shall come, ye may remember that I told you of them. And these things I said not unto you at the beginning, because I was with you. But now I go My way to Him that …
Alexander Maclaren—Expositions of Holy Scripture: St. John Chaps. XV to XXI