A given exception to the rule.
It is only my opinon as I do not know it as fact...
I would imagine because the holder is the ONLY position/player in football that receives the ball in his starting positon of being on one knee.
The other positions/players start out in other positions, make moves and than their knees go down.
So even if a WR goes out for a route, knees down and catches the ball, being on his knee was not his starting positions.
If a QB falls back and goes down on a knee it was still not his starting position.
However the holders natural starting position is on one knee.
I think that makes sense anways.