I think it's because the other parts of the virus are more likely to change over time, because there's lots of neutral mutations available: changes which don't affect fitness but do affect antibody response. The RBD by contrast is much more constrained: most mutations there result in a virus which can no longer infect cells, so it's much less likely to change (though of course any mutations which increase its effectiveness will be heavily selected for). This I think was one of the main reasons the spike protein was targeted by the mRNA viruses.