Why is boost limited at higher rpms?

[8600RPM]

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?

[kitw]

smaller turbos spool faster.
larger turbos flow more.

We have small turbos that can't flow enough to sustain the pressure ratios desired at high RPMs. These cars run a LOT of boost.

[dapple]

The turbos are reaching their peak efficiency at lower RPM's.

[Bowser330]

Yes you are correct the turbos cannot keep up with the engines air requirements for the power to keep increasing safely and efficiently.

As the others have mentioned the turbos are small and a really good point is the efficiency. Every turbo has an efficiency curve, smaller turbos has higher efficiency at lower rpm and lower efficiency at higher rpms, lower efficiency essentially means more heat so the compressor side is heating the air and as you know hot air is less dense so carries less oxygen. the cars sensors will read the extra heat and the tuner will design the the tune so that boost is reduced to prevent that extra heat since the heat can cause predetonation or knock as well as the fact that he hot air has lower power adding ability since the hot air has less oxygen.