Why can God now pour out living water on the human race?
Because Christ, the spiritual Rock, was smitten for our transgressions and crucified once. The work of the cross was completed once for all, unlike the repeated high-priestly sacrifices, so God can now pour out living water on the human race.
Les teaches that Israel ate the same spiritual food, the manna, and drank the same spiritual drink, for they drank of that spiritual Rock that followed them, and that Rock was Christ. When Moses took the shepherd's rod and smote the rock, he was smiting Christ, the smitten Christ. Isaiah 53 speaks of Christ being smitten for our transgressions, referring to the cross. As a result of the work of the cross, God can now pour out living water on the human race. Les emphasizes that Christ was crucified only once, and Hebrews makes it emphatic: this He did once, never more than once. The high priests did their work over and over, but Christ entered in once.