Because he does nothing to increase the visibility of the brand among the customers (the general recreational golfing population) that TMAG is trying to bring---and hold to---its product.
Former World Number one or not, the typical casual golfer wouldn't be able to pick Luke Donald out of line-up. Luke Donald is the kind of player that only a very serious player and someone who follows the Tour closely would recognize....or feel any interest in what equipment he plays.
Which is PRECISELY the kind of player that a Mizuno or a Titleist would be looking to bring to their brand.
If this were a rumor, that it was Titleist that was trying to win Donald away from Mizuno...I'd take it seriously.
But Taylor Made? No.
Mizuno and TMAG are fishing at different depths and at opposite ends of the golf equipment market.