Hi JSeward,
I think it is because the pressure drop in the throat of the carburettor is proportional to air density * V squared (of the air flow through the carb).
The fuel flow is dependent on the pressure drop.
However the mass flow of the air into the engine is proportional to air density * V.
Therefore on simple carburettors some device to control the fuel flow (like a tapered needle) is used to match the air mass flow.
The tapered needle will only be correct for one air density, and when operating at different density altitudes, then it is necessary to "tweak" the mixture using the mixture control.