Avoiding excessive subordination. When too many subordinate clauses are strung together, readers may have trouble keeping track of the main idea expressed in the independent clause.

too much subordination

Philip II sent the Spanish Armada to conquer England, which was ruled by Elizabeth, who had executed Mary because she was plotting to overthrow Elizabeth, who was a Protestant, whereas Mary and Philip were Roman Catholics.

revised

Philip II sent the Spanish Armada to conquer England, which was ruled by Elizabeth, a Protestant. She had executed Mary, a Roman Catholic like Philip, because Mary was plotting to overthrow her.

Putting the facts about Elizabeth executing Mary into an independent clause makes key information easier to recognize.