Because, according to the message Matthew Levi brings Woland in chapter 29, the Master "did not earn the light, he earned peace." The book does not explain the reason, but what it shows suggests that the novel earns him rest, and that the retreat before persecution, which led him to burn the book, keeps him from the light.
The scene is brief. Levi says that "He read the Master's work" and asks Woland to take the Master with him and return his peace to him. Woland asks why they do not take him "to the light," and Levi answers with the sentence of the verdict. Chapter 29 does not say who this "He" is; in chapter 32, Woland calls the couple's destiny "what Yeshua asked for you."
The yardstick is cowardice. Before dying, Yeshua said that the worst of human weaknesses is cowardice, and the book hands out the final destiny by that measure. Pilate, who condemned an innocent man out of fear, remains for about two thousand years bound to the interrupted conversation. The Master, who burned the novel gripped by a fear that, in his own words, was no longer that of the articles, and who refused to let Margarita share the ruin with him, receives rest.
The refuge is concrete: a house with grapevines, Schubert's music at night, candles, and goose quills for writing. Margarita describes it as "what they did not give you in life, the silence." It is less than light, but Woland presents it as a gift, and Margarita speaks of the eternal home "that was given to them as a reward."