Why do we celebrate Lent for 50 days?

Why do we celebrate Lent for 50 days as compared to the Latin Church who celebrates Lent for 40 days? Great Lent itself lasts for forty days, but unlike the West, Sundays are included in the count. Great Lent officially begins on Ash Monday, seven weeks before Easter and runs for 40 contiguous days, concluding with the Pre-sanctified Liturgy on Friday of the Sixth Week. The next day is called Lazarus Satur- day, the day before Palm Sunday. However, fasting continues throughout the following week, known as Passion Week or Holy Week, and does not end until after the Paschal Vigil early in the morning of Easter Sunday.