The main reason Lord Krishna did not marry Radha was a curse by Sridhama, a friend and devotee of Lord Krishna. But per another story, the original Radha and Krishna secretly married in the Bhandervan forest in the presence of Lord Brahma. It was Chhaya Radha who married Ayan, who was a partial incarnation of Lord Krishna.
Other than the reason mentioned above, there are many other reasons given by different sources that speculate why Lord Krishna did not marry Radha.
1. Sridhama’s Curse (BVP Ch. 3, 91-100):
According to the Brahma-Vaivarta Purana, once Lord Krishna was enjoying Rasalila with Radha in Goloka but left her in mid and went to another gopi named Viraja. He enjoyed amorous past-times with her. One of Radha’s friends told her about this. Therefore, she stomped the palace of Krishna along with other gopis where they were stopped by Sridhama, who was guarding the palace.
Seeing angry Radha, Lord Krishna disappeared. Hearing the word Radha and witnessing the disappearance of Lord Krishna, Viraja turned herself into a river. Therefore, Radha could not find her and returned to her home.
But Lord Krishna could not bear the separation from Viraja and asked her to take her previous form. He impregnated her, and after 100 years of celestial pregnancy, she gave birth to seven sons.
When Radha came to know about this, she was infuriated and spoke harsh words to Lord Krishna. Seeing this, Sridhama could not control his anger and spoke harsh words to her. Sri Radha could not control her anger and cursed him, “O lowest wretch in Vraja, Sri Krishna is your master and not Mine? You praise your father and rebuke your mother. As the demons always insult the demigods, so you always insult Me! Therefore, O fool, become a demon! Gopa, leave Goloka and go to a demoness’ womb. Fool, now I have cursed you; who can protect you?”
After hearing her words, his lips trembling in anger, Sridhama cursed her: “Go to the womb of a human woman! You are angry like a human woman; therefore, become a human woman on the earth! Mother, I have cursed you. Of this, there is no doubt. you will be eclipsed by a part of your own shadow. The fools on the earth will say you are the wife of a petty king. That petty king will be a partial incarnation of Lord Hari. He will be a great yogi born in a human womb because of Radha’s curse on him. In Gokula, you will attain Lord Krishna. You will stay with him in the forest there, and enjoy pastimes with Him. You will be separated from him for 100 years, and then you will attain him, your Lord, again and return with him to Goloka.”
In the Varaha-Kalpa, Radha went with Lord Hari to the earth. She attained birth in Gokula in King Vrsabhanu’s home. Because of the curse of separation for 100 years from Lord Krishna, he did not marry Radha or even went to meet her in Vrindavana.
2. Radha and Krishna are not separate.
Many people believe that Radha and Krishna were not two different personalities but a singular entity. Therefore, they did not need to marry. How can one marry himself/herself?
Goddess Radha is the left portion of the Supreme Lord. Therefore, they are inseparable. This is a widely accepted answer to the question.
3. Radha was already married.
Per some stories, Radha was 1-5 years elder than Lord Krishna. By the time they met, Radha was already married to Ayan. Therefore, he could not marry Radha.
4. Human norms did not apply to them.
Radha and Krishna were not common human beings but were divine beings. Therefore, per some opinions, the traditions like marriage and other social norms did not apply to them. Their love was considered to be divine and beyond the physical realm. Therefore, they did not marry.
Conclusion:
Among the above reasons, I think the first reason is correct because Lord Krishna did not only never meet Radha but also Yashoda and Nanda, his foster parents (Per some stories, he met them after a very long time). It seems he feared meeting Radha and breaking the curse.
Brahma Vaivarta Purana and Garga Samhita mention that Lord Krishna met Radha and other gopis after 100 years of separation because of the curse. Therefore, we can say that Lord Krishna did not marry Radha because of the Sridhama’s curse.