Why did Christoph Willibald Gluck reuse music from earlier works in his London operas?

Christoph Willibald Gluck reused music from his earlier works for his London operas, La caduta de’ giganti and Artamene, primarily due to a lack of time. He had been invited to London to challenge the dominance of George Frideric Handel, but his arrival and the subsequent production of these works were complicated by the political instability caused by the Stuart rising, which had forced the temporary closure of London theaters.1