When thinking about election and reprobation, we must remember that because all men fell with Adam, God is not obligated to save anyone. When you give a gift to a family member, other people do not have the right to demand that you therefore give them a gift as well. Salvation is a gift and God dispenses it as He pleases.
The Potter, says Paul, has rights over the clay (Romans 9:21). He has the right to save whom He will and pass over whom He will. If His goal were to promote pots in general, then creating some destined for destruction would be silly. But His goal is His own glory, so He uses some pots to show His grace and some to show His justice. In all this He not only maintains His righteousness, but also manifests it.