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Why did Diodorus Cronus reject the possibility of unrealized possibilities?

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History of logic Zeno's paradox, illustrated by Achilles' racing a tortoise.

Diodorus Cronus rejected the existence of unrealized possibilities because of his Master Argument, which posited that the following three propositions are jointly inconsistent:

Everything true about the past is now necessary.
The impossible does not follow from the possible.
There is something that is possible, yet neither is nor will be true.

Because Diodorus was unwilling to abandon the first two premises, he was forced to reject the third. Consequently, he concluded that whatever is possible must either be true now or become true in the future. In his view, there are no possibilities that remain permanently unrealized; if a state of affairs is possible, it must eventually be actualized.1