Why not all, then? Why only save some? Why this person, and not that person? You haven't answered my question, so...
If another voice helps... For me: Because in the same way 10 signs/judgement hardened Pharaoh's heart, those same ten turned the heart of Israel to Him. The same Lord and Savior that turns our hearts, turns other hearts away. The same rain that falls, causes some to be thankful, and others to curse or ignore etc. Romans 1:21
If it helps, I think all of us, equally see Pharaoh hardened and the Israelite turned and that God knew His actions would do these. "He causes the rain to fall on the just and the unjust." It is therefore, God's good actions, always, that turn the heart or soften it. In Calvinism, man's heart 'should' turn from Him because of the lack of God, therein (or we'd be in God's grace already). The same question you ask, we ask too, but we are simply seeing that nobody gets away from it: the implicating questions are ever there, whether one is a Calvinist, Arminian, Open Theist, or other. As far as I've ever been able to tell, barring Process Theology, God acts and the outcome of saving some is the same, and never, under any system, satisfactorily address to where one is no longer troubled. We simply do not know. The Calvinist does rely on Romans 9, in the sense that we don't really have a 'right to know' being that all men are God's and His alone. I do rest in the fact that He loves all and is not willing that any should perish. If it isn't "Calvinistic" at that point in sentiment, it isn't in Arminian or Open Theism either.
Allow me to reword it, because it is foundational to answering your question:
Would you agree that logic would follow that if God simply decided, before the foundation of the earth, whom He would regenerate, and would not regenerate, arbitrarily, that it would make Him inherently unjust, and therefore not worthy of our worship?
As with above, not if the act and the Actor are inherently good. That is, if rain, that is 'supposed' to soften, hardens instead, then the 'intent' is for good, albeit in this case, it is known that it won't do that. The problem, as I see it, is in the questioner's mind at that point. They are assuming 'rain for this person is bad' because the result is damnation. That is true, but the thing that caused it isn't the 'good thing' in and of itself. You and I both know that our Lord Jesus Christ is nothing but good, yet scripture says men will stumble because of Him (that's why I say it is neither Calvinist nor any other theology that adheres to this, we all do).
None deserve His mercy or His grace.
But my question is challenging exactly what you are proposing God did.
I've always wrestled with this, even under Arminian thought. Becoming a Calvinist didn't really 'mean' anything or touch the concern you bring. I always had it and even prior to Calvinism embrace, there was no theology that made it go away. Rather, the scriptures that God is good and all that He does is good, as applied to 'hardening Pharaoh's heart (Romans 9) or our Lord and Savior causing men to stumble, was instrumental in helping me get beyond this. It isn't that God 'desires' Jesus to cause men to stumble. This isn't the 'desire.' It is, however the known result. Even in Open Theism, God knows the numbers and reality. It just doesn't make a difference, at this point, whether He does or doesn't know their names. The problem, for me, is/was the same. He is good. Some are saved by His good.
-Lon