How could the Son of God, God Himself, come into time and space, take on human flesh, walk the dusty roads of our world, proclaim the gospel of salvation, go about healing and doing good, manifesting the glory of God in flesh so we can really see. And how can, in spite of this majestic display of God's goodness and His desirability, the answer to every need of the human heart, how can mankind categorically reject Him?
That's hard to BELIEVE!
That's one side of that double meaning (double entendre of "Hard to Believe")
The second is that, the reason man rejects Him, and part of what John is saying here in the prologue to his gospel, is… "It is HARD to believe!"