Except He did explain it to the Apostles.
John 6:
60 Many of his disciples, when they heard it, said, "This is a hard saying; who can listen to it?"
61* But Jesus, knowing in himself that his disciples murmured at it, said to them, "Do you take offense at this?
62* Then what if you were to see the Son of man ascending where he was before? 63* It is the spirit that gives life, the flesh is of no avail; the words that I have spoken to you are spirit and life.
This passage does not speak to the Real Presence - which is spiritual nourishment, in the substance of the Holy Eucharist.
Oh wait, so the words are the spirit, oh wait, the water, no, no the blood, no, the flesh...no, you just have to believe, etc., etc. Gee!
And then it says a couple of verses further (Jn. 6:65): "And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father."
So, all this eating and drinking is useless...
Now maybe you can begin to see why they say "John's" Gospel is heavily interpolated.