Why was Lady Jane Grey desposed?

Answer 1

Short answer: Mary Tudor had a better claim to the throne.

Long answer: When King Henry VIII died, he left three children: Edward, Mary, and Elizabeth. Edward, as the only son, became king; if Edward died without children, it was understood that Mary, as the elder daughter, would take the throne after him.

Edward VI died at age fifteen. He did not want Mary to take the crown for fear that she would restore Catholicism (he, unlike Mary, was a Protestant). So in his will he arranged for Mary and Elizabeth to be excluded from the line of succession on the terms that they were both bastard princesses. The crown then fell to the conveniently Protestant Lady Jane Grey, the granddaughter of Henry VIII's younger sister.

Lady Jane Grey, though she was reluctant to do so, was pressured by her husband's father, the Duke of Northumberland, to accept the crown. Mary gathered military support and defeated the Duke of Northumberland's supporters. It was then a simple matter to depose the defenseless Lady Jane Grey.