Why aren't Radha and Krishna married?
Hare Krishna! Thanks for your question. It is a very elevated and complicated topic. Here's one answer:
They ARE married -- in some parts of the lila ("pastimes of the Lord") and in some parts of Vrindavan. But He also wants to have the most intense possible relationship with Her, and therefore His yogamaya (Krishna's spiritual energy) arranges for many stimulating "obstacles" in the core of His spiritual realm, which cause Their emotions to peak unlimitedly.
The understanding is, as they say in America, "absence makes the heart grow fonder"! When we're afraid of losing something, or something we want is not permitted, we want it all the more. Ask any teenager - whatever a parent says is "off limits," that's what they want. In the spiritual world, this principle is heightened. In order to increase the intensity of love between Radha and Krishna, this threat of separation - and also the intensity of longing - are added to the loving relationships between Radha and Krishna to inspire deeper affection and deeper emotions.
I hope this is helpful.