Because of the reduction in the density of the atmosphere associated with an increase in altitude:

The reduction in atmospheric density with an increase in altitude affects how air and fuel mix in the engine. As altitude increases, there is less air density, meaning there is less oxygen available for combustion. To maintain an optimal air/fuel ratio and prevent an overly rich mixture, the mixture control needs to be adjusted towards a weaker position. This adjustment allows for the proportion of fuel to air to remain balanced despite the decreased amount of oxygen, ensuring that the engine continues to run efficiently at higher altitudes.

Adjusting the mixture towards the weak position helps compensate for the thin air, preventing the engine from flooding and enabling it to achieve the desired performance levels. In contrast, moving the throttle to maintain the best air/fuel ratio or enriching the mixture would not account for the reduced oxygen availability, and increasing the octane rating does not directly address the issue of air density. Thus, adjusting towards the weak position is the logical and correct approach to managing the air/fuel mixture in response to altitude changes.