For the same reason an ice skater spins faster when her arms are pulled in the earth's rotation slows as the moon moves further away.The total Moon Earth Angular momentum is constant. This particular example however has many different solutions, for example if the spin inreased 21 hours it would also work. For that matter if you and or subtract any integer number of days it too would work. However Apollo left some mirrors on the moon that when lasers are focused on it gives a return beam that allows measurement to the moon within mere millimeters, and those measurements are increasing. Probably consistant with the numbers in this article too. (Tidal forces have already slowed the moon until it shows us the same face, it will eventually slow the earth so the moon sees the same face too)
For the same reason an ice skater spins faster when her arms are pulled in the earth's rotation slows as the moon moves further away.The change in the Earth's rate of rotation is attributed by that scientist to isostatic rebound due to the loss of the glacial ice thousands of years earlier, which fits with the ice skater example.