When Italian operas fell out of fashion, he started composing oratorios, including his most famous, Messiah.

Why did Handel change from opera to oratorios?

Handel tended more and more to replace Italian soloists by English ones. The most significant reason for this change was the dwindling financial returns from his operas. Thus a tradition was created for oratorios which was to govern their future performance.

Why was oratorio created?

Protestant composers took their stories from the Bible, while Catholic composers looked to the lives of saints, as well as to Biblical topics. Oratorios became extremely popular in early 17th-century Italy partly because of the success of opera and the Catholic Church's prohibition of spectacles during Lent.