Why did Elizabeth I imprison Mary Queen of Scots?

Answer 1

Although everyone assumed Elizabeth would marry and produce an heir to the English throne, she remained a "Virgin Queen." Despite her many suitors, including King Phillip II of Spain and Robert Dudley, she remained unmarried and childless. This sparked great concern among her advisors, as Mary Queen of Scots was a likely successor. Her Catholicism was a major threat to England and, as such, she was taken prisoner for 19 years under Elizabeth's reign. In 1587, Elizabeth's counsellors finally convinced her to execute Mary on charges that she had plotted to overtake the throne.

Answer 2

Mary Queen of Scots was imprisoned by Elizabeth the I under the impression she was comitting treason. That she was going to assinnate Elizabeth, or tried to already, in order to get the English throne as well as the scottish which she already posesed