[Moppie]

In the simplist possible terms?

Airflow and phyiscal strength.

And engine needs air to rev, the more air that can be fed into it, the higher it can rev.
Air flow is of course dictated by intake design (from intake through manifold, head ports and past valves), camshaft profile and timing and bore/stroke ratio. And of course fuel supply.

Phyiscal strength depends on metals used, hardening techniques used on the metals, and of course physical design.

Manufactors build a rev cut into modern engines at a safe point before one of the above is reached, and in most cases if this is by passed most enignes will run out of air or fuel before reaching thier strength limits.
Running out of air or fuel can cause lean or rich burning which can damage an engine, and of course reaching the strength limits of internal parts can be catasrophic.


To make an engine rev higher you have to increase the air/fuel supply and or make the engine internals stronger.