The simple answer is that, given their greater theological systems, they cannot affirm that God loves all people and that he works for their salvation above everything else. Their theology is deterministic, by which I mean that the divine will is logically prior to human will in everything. God doesn't merely foreknow what people might do in the future; he effectively decides what they will do in the future. God's relation to the world is like the relation between an author and his book: he knows his characters' history because he decides it, not because he reads about it or learns about it from someone else. And of course, they believe that some persons are going to be damned forever. Thus the damnation of some persons is ultimately traceable to an inscrutable decision of God from all eternity that it was better (or at least good enough) for things to turn out that way.
Perhaps God decided this because he wanted to demonstrate his justice in the natural order through their punishment, as Thomas Aquinas suggested. Maybe he simply decided that it would be good for things to happen like this, for some reason which is certainly good but which we cannot know or comprehend. In any case, it is clear that on their system, the salvation of the human person as such is not God's foremost goal or end. Something else is -- his glory, perhaps.
This is why I don't count them among my theological influences. For me, this sort of theology is a step too far. It is very difficult to conceive of a good antecedent reason for which God would determine to create some persons only to damn them in the end. On the contrary, the very idea of it sooner suggests malice or plain nastiness beyond what is conceivably good.
Isaac the Syrian would point to the patience which God has with sinners on this earth as an evidence that, as far as the divinity is concerned, the goal of everything is their salvation. God wants nothing else than that people be saved, all of them. But there is a counterargument to this: he is patient with the reprobate (those foreknown to damnation) here only so that it will serve to exacerbate their guilt after death. Isaac's response is this:
If someone says that He has put up with them here (on earth) in order that His patience may be known -- with the idea that He would punish them there mercilessly, such a person thinks in a unspeakably blasphemous way about God, due to his infantile way of thinking: he is removing from God His kindness, goodness and compassion, all the things because of which He truly bears with sinners and wicked men. Such a person is attributing to God enslavement to passion, supposing that He has not consented to their being chastised here, seeing that He has prepared them for a much greater misfortune, in exchange for a short-lived patience. Not only does such a person fail to attribute something praiseworthy to God, but he also calumniates Him (Second Part 39, §2).And elsewhere, rejecting the notion that God has any concern for retribution, he writes:
That we should imagine that anger, wrath, jealousy or the such like have anything to do with the divine Nature is something utterly abhorrent for us: no one in their right mind, no one who has any understanding at all can possibly come to such madness as to think anything of the sort about God. Nor again can we possibly say that he acts thus out of retribution, even though the Scriptures may on the outer surface posit this. Even to think this of God and to suppose that retribution for evil acts is to be found with Him is abominable. By implying that He makes use of such a great and difficult thing out of retribution we are attributing a weakness to the divine Nature. We cannot even believe such a thing can be found in those human beings who live a virtuous and upright life and whose thoughts are entirely in accord with the divine will -- let alone believe it of God (ibid.).Isaac appeals to what we can plausibly think true of a person we would call good. Given your understanding of what a good and godly person is like, can you conceive of a godly person knowingly and intentionally determining another person to commit crimes which ultimately will lead to the latter's death and demise? Does that make any sense at all, given your understanding of "goodness"?
In my article "Christian apokatastasis: Two Paradigmatic Objections", I make the point that goodness is not properly defined as obedience to obligations or duty or anything of the sort. On the contrary, goodness is a kind of disposition to act for the benefit of the other person, especially when she is undeserving. A good person is one who is disposed to act for the benefit of another. Once this point is appreciated, it becomes glaringly obvious that God, if he is genuinely and essentially good, could never predestine anyone to damnation. Damnation can only come through the free agency of the human creature, rather than from the side of God.
Isaac's got a different idea about God's character:
Among all His actions there is none which is not entirely a matter of mercy, love and compassion: this constitutes the beginning and the end of His dealings with us (39, §22).That's why I don't like Augustine and Calvin: because on this issue, which is really an important and central one, we have such radically different conceptions of what God is like. There also other issues, too, but this is the biggest one.