How and why does Durkheim view science as closer to the truth?
I'll try to answer your question although it does not make clear what is the second term of your comparison. Science is "closer to truth" than what? Religion?
Durkheim argued that human society works as a social system, following the same model of a biological system. Because of that, social facts can be studied by applying the same scientific techniques that are used to explain scientific facts. Thus, in The Elementary Forms of Religious Life, Durkheim sets out to study religion with a rational and scientific attitude, rather than with a theological and metaphysical approach. He described religion as an institutionlised attempt to integrate the community and achieve solidarity within it. Studying totemism within the Arunta, Durkheim concluded that the existence of the totem in every band made the members aware of their existence within a larger clan and thus provided them with a sense of allegiance to other bands that would have otherwise been lost.