Because drunken drivers are severely impaired, a couple moments at a rest stop isn't going to reverse that situation.  A drugs in their system.  Being tired is being tired.  I'd estimate that the majority of people at one point or another have been or found themselves tired behind the wheel and thought I better get (wherever they're going) so they can get off the road and rest.  The problem is that can cause accidents too, most people wont just stop on the side of the road anywhere to sleep.  Unless you're from the lower 48 where that's more common especially among the truck driving community.  

that's why theirs a difference.  There is a ticket for driving dangerously, Ie: driving recklessly, etc. and most likely driving while tired falls under driving reckless but you aren't under the influence of anything in your system, common sense