Because they were closest to the action. Obviously. Just like Joe Stalin said, it depends on who does the counting. You don't like my sampling of those up close and personal, then how about the Warren Commission:
"What follows is the result of a survey of the 121 witnesses to the assassination of President Kennedy whose statements are registered in the twenty-six volumes appended to the Warren Report.[1] On the question of where the shots that killed the President came from, 38 could give no clear opinion and 32 thought they came from the Texas School Book Depository Building (TSBDB). Fifty-one held the shots sounded as if the came from west of the Depository, the area of the grassy knoll on Elm Street, the area directly on the right of the President's car when the bullets struck."
http://spot.acorn.net/jfkplace/09/fp.back_issues/12th_Issue/51_wits.html
One of those counted as a grassy knoll witness in the above listing by Feldman is the Secret Service agent Forrest Sorrels, thusly:
In the lead car rode two Secret Service men, Winston G. Lawson of the White House Detail, responsible for planning many details of the motorcade, and Forrest V. Sorrels, in charge of the Dallas office. Lawson looked back at the President's car continually in order to regulate the speed of the procession. When he heard the shots (car #1 was almost at the Triple Underpass when he heard them) he was positive that they came from the rear of his car, and when he saw an agent standing up with an automatic weapon in his hand in car #3, Lawson's first thought was that the shots came from that gun. (IV, 352-3)
Sorrels remembered scanning the Depository as his car turned the corner, but he saw "no activity, no one moving around that I saw at all." As soon as he heard the shots, he "turned around to look up on this terrace part there, because the sound sounded like it came from the back and up in that direction." He had to repeat this for the Commission's assistant counsel, Samuel A. Stern.
And, as I said, the noise from the shots sounded like they may have come back up or the terrace there ... But the reports seemed to be so loud that it sounded like to me --- in other words, that my first thought, somebody up on the terrace, and that is the reason I looked there.
As noted by the author, the car Sorrels was in was just about to enter the triple underpass, and he thought the shots came from the terrace. Sorrells is counted by Feldman - and no doubt by our resident CT Robert Prey - as a grassy knoll witness.
Even non-conspiracy sources like John McAdams tabulation counts Sorrels as a knolll witness.
See here:
http://mcadams.posc.mu.edu/earwitnesses.htm
But that's not the whole story.
Well, let's put this in context, and look at a map, shall we?
As you can see from the map below, immediately behind and to the right of Sorrels as the lead car was about to enter the underpass was the terrace -- the Grassy Knoll. That was in the foreground. In the background and behind the terrace from Sorrels position was the School Book Depository. Forrest correctly located the source of the shots as behind him and to his right, but merely focused on the foreground object (the terrace - aka the Grassy Knoll) and not the background object (the TSBD).
It is false to count Sorrels as a grassy knoll witness when the sound could have been coming from the more distant TSBD as one can see from the map below.
Below that is an image taken from the overpass, from what appears to be directly over the center lane of Elm Street. Sorrells would have been about 20 - 25 below this camera location, and from his position, shots coming from the sixth Floor SE corner window of the TSBD would be coming directly over the terrace and sound to him as if they were coming from the terrace. The terrace, however, would be the closest object and would be looming over Sorrels. The Depository would be far more distant and almost hidden by the trees. So if the shots came from the TSBD, it is easy to see why Sorrels would focus on the knoll, and not the more distant TSBD.
I don't think this thought originates with me. I think the commission staff realized much of this, and as pointed out by Feldman, even asked if the Depository was consistent with Sorrels source of the shots. Feldman, however, paints this in a conspiratorial manner as leading the witness into conforming with the pre-conceived notion of a sniper's nest assassin:
Mr. Stern then drew him out of the area of evidence into the area of conforming opinion in order to evoke the right words from his witness:
Mr. Stern. Do you have any reason to believe that the shots could not have come from the Book Depository Building?
Mr. Sorrels. No, sir.
Mr. Stern. Would shots from the Book Depository Building have been consistent with your hearing of the shots?
Mr. Sorrels. Yes, they would have. (VII, 345-7)
Here's Sorrels complete testimony (well worth reading):
http://karws.gso.uri.edu/Marsh/WC/sorrels1.html
Note the exchange Feldman omits in his leading his readers toward a conspiratorial conclusion:
Mr. Stern. It sounded to you at first as though it came from there?
Mr. Sorrels. That is the way it sounded--back into the rear and to the right, back up in that direction. And in the direction, of course, of the building.
Note that
after Sorrels mentioned the building as the potential source of the shots, Stern then returned a brief while later to have the exchange Feldman classified as "...drawing him [Sorrels] into the area of conforming opinion." That treatment of Sorrels testimony is dishonest.
This is why it is necessary to read the original testimony, and not simply rely on conspiracy believers to spoon-feed you their interpretations.