Because middle-income clawback with sharp cliffs rather than gradual clawback starting or reaching into upper income ranges pits the middle-income segment of the working class against the poor in funding battles, helping to avoid political pressure to further increase benefits, and it also allows what can be marketed as a support system for the poor to also serve as an anchor that creates a progress wall just above the area where it provides net benefits, while minimizing the marginal impact on high-income earners.
Is this socially good? No. But it serves the interests of the people who politicians tend to see as their most important constituents, while creating a sharp division of interests between the poor and middle-income segments of the working class, obstructing the formation of working-class solidarity.