Why did Mary I rule England and Scotland?

She didn't.

Queen Mary I of Scotland and Queen Mary I of England were two different people.

Mary I of Scotland, also known as Mary, Queen of Scots was born 8th December 1542, became Queen when her father, King James V of Scotland, died on 14th December 1542 (yes, she was 6 days old). As the wife of Francis II, she was Queen Consort of France from 10th July 1559 until the death of Francis on 5th December 1560. She was executed on 8th February 1587 for supposedly plotting to overthrow Queen Elizabeth I of England.

Mary I of England was born 8th February 1516, became Queen of England on 9th July 1553 and reigned until her death on 17th November 1558.

Mary I of England was a half sister of Elizabeth I, and was known as Bloody Mary because during her reign nearly 300 people were executed for failing to follow Roman Catholicism.

When Queen Elizabeth I died she was succeeded by the son of Mary Queen of Scots. He had already been King James VI of Scotland for 20 years and he also became King James I of England - he held the two titles seperately and England and Scotland remained separate countries for another hundred years.