For the same reasons he spoke in parables he know many could not understand. Jesus knew they could never understand. No one can accept and understand unless God specifically gives them the faith to do so.
"no one can come to me unless it is granted him by the Father." Jn 6:65
"No one can come to me unless the Father who sent me draws him." Jn 6:44
In Chapter 4 Jesus gives the same figurative speech using the "living water" analogy and a whole city of Samaritans not only understand but are almost instantly converted. This despite the fact the Jews were in a much better position to understand and believe. Thus, faith is a gift from God and cannot be obtained by one's will or works.
John 6:60 is the only record we have of any of Christ’s followers forsaking him for purely doctrinal reasons. If it had all been a misunderstanding, if they erred in taking a metaphor in a literal sense, why didn’t he call them back and straighten things out? Both the Jews, who were suspicious of him, and his disciples, who had accepted everything up to this point, would have remained with him had he said he was speaking only symbolically.
But he did not correct these protesters. Twelve times he said he was the bread that came down from heaven; four times he said they would have "to eat my flesh and drink my blood." John 6 was an extended promise of what would be instituted at the Last Supper—and it was a promise that could not be more explicit.