Because the earth is orbiting the sun at the same time and in the same direction as it rotates on its axis there is one less day per solar orbit than there are rotations.
(the earth; is orbiting; the sun; L:at the same time and in the same direction; T:as it rotates on its axis) (it; rotates; on its axis)
the earth it
is orbiting rotates
the sun L:at the same time and in the same direction T:as it rotates on its axis
on its axis