The same thing that has slowed efforts to curb emissions before they need removal: the free rider problem, or collective action problem. Every country benefits

The same thing that has slowed efforts to curb emissions before they need removal: the free rider problem, or collective action problem. Every country benefits a little from one country's efforts to remove atmospheric CO2, and the non-participating countries don't pay anything. There's an incentive for every country to wait for some other country to take up the burden first. That said, I think that the cost of accelerated silicate weathering could drop significantly with further advances in automation. At some point individual governments or well funded NGOs may be able to make a dent in the global problem without waiting for everyone else to cooperate.