Because there is an object like foam roller he's sitting on and it's pushing on his perineum. It greatly reduces flow in to the penis, making his erection close to impossible. Basically like a reverse cock-ring. But instead of reducing outflow, it recuses inflow.
It releases the pressure, but it's deeply frustrating because if you remove stimulation after the point of no return, but before the actual orgasm, there is no feel of orgasm. It's called ruined because if you do it right the semen just drips away, like you are peeing. You feel nothing but slight and very temporary relief.
I know this from my own experience. When my wife suddenly stops stimulating me at just the right moment, which is frustrating, I feel a strange sensation, followed by a slow, powerful contraction. There's nothing I can do; the semen just keeps flowing out until the contraction stops. No satisfying orgasm, I remain horny. The horniness then slowly disappears over the next few minutes.