There are two explanations:
Hipparchus proposed dividing the day equally into 24 hours which came to be known as equinoctial hours (because they are based on 12 hours of daylight and 12 hours of darkness on the days of the Equinoxes).
For it is the time it takes the earth to complete one rotation on its axis. This spinning movement is called rotation. Earth’s axis is tilted 23.5° with respect to the plane of its orbit. Each time Earth rotates on its axis, it goes through one day, a cycle of light and dark. there are two kinds of time measurements one is the sidereal day and the other one is synodic day. the other one refers to the position of the sun as the earth rotates and the other one the reference is the position of the nearest star as it rotates. the distances between the said star and the sun is almost 4mins. To complete one revolution of the earth is 365.25 days so if your going to divide it into 24 hrs. still there will be an excess that why leap year happen. the earth rotates 24 hrs. not 48 hrs because, the earth imaginary centripetal and centrifugal force is responsible for it to maintain the earth on its orbiting position.