The crowds were looking for a Christ who would “remain” and lead them into glorious earthly kingdom, but Jesus was talking about being “lifted up” and not being with them forever. The Jesus they wanted wasn’t the Jesus they were getting.
When I was 5 my grandfather died. As I stood by the graveside at the cemetery, my mother said I looked down into the grave and then turned to her and asked "How's Grandpa ever going to get out of there?"