When you have upper level divergence aloft, you will be pulling air from below upwards to replace the air that had left. As long as this area of upper divergence persists, the air will continue to rise from below this area. Therefore the surface pressure below will remain low. Without the upper level divergence, the surface low will quickly fill as the surrounding air will converge into this area of low pressure.
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When you have upper level divergence aloft, you will be pulling air from below upwards to replace the air that had left. As long as this area of upper divergence persists, the air will continue to rise from below this area. Therefore the surface pressure below will remain low. Without the upper level divergence, the surface low will quickly fill as the surrounding air will converge into this area of low pressure.