Why is boost limited at higher rpms?

Can someone explain to me the science behind having to limit boost at the higher rpm range? All the tunes that also showed their boost level clearly drop near the top end. I am naïve to turbo's, I tried to read on the net but I am not understanding it well. To me it seems like there would be a ton of exhaust gas at higher rpm driving the turbos and what stops it from sustaining the same boost as the low-mid range?



Can it literally not keep up with the engine's air requirements?