Agree that Krishna and Radha were in eternal love. Lets clear a point about the meaning of love in this context. Love doesn't mean that you have to get married. Marriage is just a symbolization. In Fact, marriage is a material thing and Krishna doesn't give a darn to material things. Krishna and Radha were attached to each other mentally. That's why we still say Radhe-Krishna and not Rukmini-Krishna. Rukmini had done a lot of work to acquire Krishna. She had went against her brother Rukmi. Rukmani, like Radha was in love with Krishna. Rukmini had sent a letter of plead to Krishna that He should come and take Rukmini. The letter was composed of 7 glistening shlokas. They touched Krishna's heart and he had to accept Rukmini's plead and make her, his first wife. The marriage of Radha and Krishna, on the other hand doesn't really matters. Again, because it is a material thing. And what doesn't matters, there is no need to ask why it didn't happen. Because, there is no need of it happening. Plain Answer: There was no need of the marriage of Radha and Krishna. As you rightly pointed out in your question, they were bound by eternal love. Marriage is not required. Now something more We know that Lord Krishna and Radha are eternal lovers who dwell in Go-loka. In Goloka their pastimes are always going on. So as per BrahmaVaivarta Purana, in Goloka, before the manifestation of their pastime on earth, on one occasion Radharani had a heated argument with Shridama, the personal attendant of Krishna. In course of their conversation Radharani got angry and cursed Shridama to take birth as a demon. So Shridama also in return cursed Radharani to be born as a human, and after spending pastime with Krishna in Gokula She will get separated for 100 years from Her beloved lover Krishna: manuṣyā iva kopaste tasmātvaṃ mānuṣī bhubi bhaviṣyāsi na saṃdeho mayā śaptā tvamambike [BVP - 4.3.103] Meaning You are getting angry like a human, hence you'll become a human. Mother! you'll become a human without any doubt due to my curse. gokule prapya taṃ kruṣṇaṃ vihariṣyasi kānane bhavitā te varṣaśataṃ vicchedo hariṇā saha punaḥ prapya tamīśaṃ ca golokamājamiṣyasi [BVP - 4.3.105,106] Meaning In Gokula getting the company of Krishna you will enjoy pastimes in the forest. Then you'll experience a separation of hundred years from Hari. After that you'll again get His company and return to Go-loka. Hence, even though Krishna's love for Radha was eternal, due to the effect of the curse He couldn't marry Radha socially or openly. Instead, as per the curse, on Earth Radha had to marry Rayana of Vrindavan who was also Kirshna's partial incarnation: mūḍhā rāyaṇapatnīṃ tvāṃ vakṣanti jagatitale rāyaṇaḥ śrīhareraṃśo vaiśyo vrundāvane vane [BVP - 4.3.104,105] Meaning On Earth fools will say you to be the wife of Rayana. Rayana will be the partial incarnation of Hari in Vrindavan. However, although Krishna didn't marry Radha on Earth formally or socially, all the gopis including Radha were in fact His wives in Go-loka. Hence, during their pastimes in Vrindavan they were wedded to each other by Brahmā himself and Krishna had spent time only with His wives from Go-loka: tābhiḥ sārdhe sa reme ca svapatnībhirmudā'nvitaḥ pāṇiṃ jagrāha radhāyāḥ svayaṃ brahmā purohitaḥ [BVP - 4.115.88] Meaning Lord Krishna enjoyed pastimes in Vrindavan only with those of His wives who were residents of Go-loka. Brahmā himself as the priest wedded Radha to Him. Once Radha asked Krishna why He loved Radha but married others. Lord Krishna gave a sweet reply of Radha's question. Lord Krishna said that marriage is only possible between two persons but not between one soul. We (Krishna and Radha) are one soul so our love will be eternal and divine but earthly marriage is not possible. But Krishna made a promise to Radha - whenever anyone in this world will remember Me, they will always take your name before Mine, and never any of My wives, as a reminder of our unconditional and immeasurable love.