I posted the reason the other day and many people read it and no one responded so I deleted it. I will write it again.
lets be extreme, that is the best way for illustrations. In this example we will have two backswing positions, One is completely upright where the shaft is vertical, the grip is in a straight line from the clubhead to the ground(we will call this 0 degree backswing). the other position is horizontal whaere the clubhead and grip are both the same distance above the ground(call this 90*)
if we reach the top of the backswing at 0* and start our transition, the club can litterally drop at the the speed of gravity, and your hands will not be moving much faster then this, if it all. so no force, or very little is required by you to get the club in the slotted position. Now in the 90* position the club cannot drop in the transition, if it does then you are very screwed. Also, our muscles must be tighter to stop the clubhead from falling closer to the ground and any movement back towards the ball will be ONLY directed by the person swinging the club. when we make the club move it will start releasing shortly after, unless we hold back which would not be ideal in the golf swing. So the more vertical we are at the top the easier it is for the club to drop and the further we can get in the downswing untill our force starts releasing the club. Now it does not mean we should swing at 0* but there is a point where we get too close to 90 and from there it woud be very difficult to recover. One solution to a really flat backswing would be to let the left elbow bend a little, but that is an adjustment that should not be necessary in a golf swing.