Why is the ambient pressure affecting the allowable MAP for a given temperature?
Cos Frank says so.
In fact, as density altitude increases, the power produced for a certain manifold pressure decreases because the mixture gets richer. Restricting the MAP cripples the aircraft even more.
The only possible explanation I can come up with is that engine cooling is less efficient.
Other Lycoming equipped aircraft do not have this limitation.
I suspect this is one of the reasons Robbies generally make TBO and others do not.