I think a key difference between the Republicans dragging Dems to the right and our attempts to pressure them leftward is that Republicans are a ruling party, represented by the land-based/natural-resource-based wing of the US bourgeoisie. There is not yet a Marxist party, rooted in the US working class, capable of doing the same. We have to build and organize that structure first, which requires putting forth demands that address the immediate concerns of the working class. While there are radicalizing layers of workers and students gravitating towards the abolish demand, a "maximalist" demand like that might seem like a bridge too far for most workers. The defund demand addresses the same concern but reaches a wider audience and can be articulated as a first step towards the eventual goal of abolition.