Because it can be assisting or interfering with play

The Official Rules of The game of golf appear on the About.com The game of golf site courtesy of the USGA, are used with permission, and will not be reprinted without the permission of the USGA.
A golf ball on the putting green may be cleaned when lifted under Rule 16-1b. Elsewhere, a ball would be cleaned when lifted except when it has been lifted:
a. To determine in case it is certainly unfit for play ,
b. For identification, in which case it would be cleaned only to the extent necessary for identification; or
c. Because it can be assisting or interfering with play.
In the event that a player cleans his golf ball during play of a hole except as supplied in this Rule, he incurs a penalty of one stroke and the ball, supposing that lifted, must be replaced.
If a player who is usually required to replace a ball fails to do so, he incurs the penalty for breach of Rule 20-3a, but there is no additional penalty under Rule 21.
Exception: In case a player incurs a penalty for failing to act in accordance with Rule 5-3, 12-2 or 22, there is no additional penalty under Rule 21.