The reason for this 'delayed' or 'deferred' Moksha for Jay and Vijay is actually a very interesting conundrum and I have found the resolution to this in only one scripture which is the Vishnu Puran.
Maitreya actually asks the very same question to Rishi Parashar wondering why the same being, in the forms of Hiranyakashipu and Ravan, though having been killed by the hands of the incarnations of Lord Vishnu, did not achieve instant deliverance but as Shishupala he was granted that salvation instantly.
I am sharing the relevant text below:
Maitreya said:— Most eminent of all who cultivate piety, I am curious to hear from you, and you are able to explain to me, how it happened that the same being who when killed by Viṣṇu as Hiraṇyakaśipu and Rāvaṇa obtained enjoyments which, though scarcely attainable by the immortals, were but temporary, should have been absorbed into the eternal Hari when slain by him in the person of Śiśupāla.
Parāśara said:— When the divine author of the creation, preservation, and destruction of the universe accomplished the death of Hiraṇyakaśipu, he assumed a body composed of the figures of a lion and a man, so that Hiraṇyakaśipu was not aware that his destroyer was Viṣṇu: although therefore the quality of purity, derived from exceeding merit, had been attained, yet his mind was perplexed by the predominance of the property of passion; and the consequence of that intermixture was, that he reaped, as the result of his death by the hands of Viṣṇu, only unlimited power and enjoyment upon earth, as Daśānana, the sovereign of the three spheres; he did not obtain absorption into the supreme spirit, that is without beginning or end, because his mind was not wholly dedicated to that sole object.
This is actually interesting since the Purana mentions that it was the death by the hands of the Lord that made Ravana attain the overlordship over the three worlds. Further it states:
So also Daśānana being entirely subject to the passion of love, and engrossed completely by the thoughts of Jānakī, could not comprehend that the son of Daśaratha whom he beheld was in reality the divine Achyuta. At the moment of his death he was impressed with the notion that his adversary was a mortal, and therefore the fruit he derived from being slain by Viṣṇu was confined to his birth in the illustrious family of the kings of Chedi, and the exercise of extensive dominion.
This gives us a really interesting concept that the demons who do not understand that they are being killed by the Supreme Lord shall probably be reborn with great opportunities but shall not attain moksha till they identify the Lord!
In this situation (as Shishupal) many circumstances brought the names of Viṣṇu to his notice, and on all these occasions the enmity that had accumulated through successive births influenced his mind; and in speaking constantly with disrespect of Achyuta, he was ever repeating his different appellations. Whether walking, eating, sitting, or sleeping, his animosity was never at rest, and Kṛṣṇa was ever present to his thoughts in his ordinary semblance, having eyes as beautiful as the leaf of the lotus, clad in bright yellow raiment, decorated with a garland, with bracelets on his arms and wrists, and a diadem on his head; having four robust arms, bearing the conch, the discus, the mace, and the lotus. Thus uttering his names, even though in malediction, and dwelling upon his image, though in enmity, he beheld Kṛṣṇa, when inflicting his death, radiant with resplendent weapons, bright with ineffable splendour in his own essence as the supreme being, and all his passion and hatred ceased, and he was purified front every defect.
Being killed by the discus of Viṣṇu at the instant he thus meditated, all his sins were consumed by his divine adversary, and he was blended with him by whose might he had been slain. I have thus replied to your inquiries. He by whom the divine Viṣṇu is named or called to recollection, even in enmity, obtains a reward that is difficult of attainment to the demons and the gods: how much greater shall be his recompense who glorifies the deity in fervour and in faith!
So as per the Vishnu Puran the reason for salvation not being achieved by Hiranyakashipu and Ravan was the lack of understanding that the being responsible for their death was Lord Vishnu Himself, as well as NOT having thought about the Lord continuously whereas Shishupal, who was born hearing about Krishna also at the time of his death, gained that wisdom of knowing Krishna as the Supreme Being Himself and attained liberation.
