why do they take soma in Brave New World?

It's important to understand that Soma, the drug that has "All the advantages of Christianity and alcohol; (and) none of their defects" is not just taken in the future world that Huxley created; it is given by the state as a combination of sacrament, aphrodisiac, and consciousness-altering substance. It's a world where government and religion have merged into a single entity, and the only real foe of such a state is dissension generated by unhappiness.
Any kind of negative emotion, be it anger, fear or sadness, would be instantly wiped out and replaced with "the warm, the richly coloured, the infinitely friendly world of Soma-holiday" where "Eyes shone, cheeks were flushed, the inner light of universal benevolence broke out on every face in happy, friendly smiles."
Every aspect of their lives is infantilized. Sex, rather than being an intimate act, is turned into a series of ritualized orgies. Instead of physical contact creating strong emotions, it served to numb intense feelings and prevent emotional connections. Taking Soma is institutionalized, and it doesn't just make people happy, it makes them "never want what they can't get." Not only do the people have no freedom, they willingly give it away. They want a dependable, easy uncomplicated pleasure that takes away all the messy and character-defining moments of being human, in part because that's all they've ever known.
Soma can be interpreted literally or as a metaphor for any number of things, including alcohol, religion, a sports team or any activity that serves as a distraction from not just the miseries of daily life, but also from the small joys that a full range of feelings can provide.

Soma is a drug that makes people feel better by altering their consciousness to a more dreamlike state. People take soma when they start to have bad feelings. It helps to take the edge off their psychic pain, and it also helps to improve pleasurable experiences. It is an important part of a world that has been engineering to reduce human suffering to a bare minimum.
We learn, more specifically, that soma helps people take a "holiday" from "malice" and "bad-temper." The drug also helps people get in the mood for the orgies that are part of the religious practice in the World State. Soma is part of the ritual preparation for the orgy.
When Bernard wants to be alone with Lenina hovering over the water in the helicopter, no other people around, Lenina, is distressed at the idea. She proposes soma as a solution:

. . .why you don’t take soma when you have these dreadful ideas of yours. You’d forget all about them. And instead of feeling miserable, you’d be jolly.

Linda, while on the Indian Reservation, uses peyote, which she likens to soma.
We can see from these examples that soma helps people feel better, helps numb people so that they don't have to think, and helps people achieve an altered state of consciousness so they can better enjoy rituals like orgies.