Because at this point in the novel, Gatsby and Nick have formed a strong friendship. Gatsby knows Nick will not rat Gatsby out and Nick would've found out eventually.
At this point in the novel not only have Gatsby and Nick formed a friendship but also a strong trust that leads Gatsby to tell Nick the truth after Nick has helped Gatsby mend his broken relationship with Daisy. He trusts that Nick will respect his wishes and keep the secret letting Gatsby Take the blame for Daisy's actions
