Continuity mistake: In the scene where Longstreet rides up to Alexander, prior to Pickett's charge, his staff stops shortly behind them, while in the next close up shot they have moved much further away to the right.
Continuity mistake: When the battle is happening at the end, the number of soldiers bodies after they have been killed varies from shot to shot. It shows them falling and dying in some shots, but the bodies are gone in later shots.
Factual error: There are several scenes which show an American flag with way too many stars for that time period.
Revealing mistake: There is one shot where the camera shows the battlefield with a large tree in the middle of it. You can see that the tree has been cut away to make room for a power line, which has been removed for the movie.
Factual error: There is one minor anachronism in the scene in which several Confederate officers are discussing politics with the British officer, Freemantle, while playing cards near a campfire. General Pickett is seen holding some paper money in one hand. The only note visible is a Confederate twenty dollar bill, with the back of the note exposed. While the note appears to be genuine, the back design is unique to Confederate twenty dollar bills issued after February 17, 1864 - more than seven months after the battle.
Factual error: As Buford's Cavalry is approaching Gettysburg there is a scene filmed from behind as Buford stops by a fence and peers out over some fields. One of the fields appears to be a corn field and the corn appears to be about 5 to 6 feet high, much too high for June 30th in that part of the country. Having been raised on a farm in New Jersey corn that was "knee high on the 4th of July" was considered a bumper crop. The height of the corn was much higher than "knee high" on what was suppose to be June 30.
Continuity mistake: There's one scene wherein Chamberlain is on the move wearing the shoulder straps of a lieutenant colonel and then, shortly later, with no explanation, on the same day, he's wearing the shoulder straps of a full colonel.
Character mistake: Chamberlain's sidearm is a Colt Army .44, which uses a peg to secure the barrel to the cylinder axle. During the fight, Chamberlain replaces the empty cylinder with a fresh one, pushes the barrel against a log to seat it, but never shoves the peg in. A loose barrel might well result in a misalignment of that barrel with the chamber being fired, which would blow the barrel off the weapon.
Factual error: Pickett's charge moved from west to east and was made about 2 o'clock in the afternoon. When Pickett rides down the line to encourage his men before the attack, it is clear from the shadows that the scene was filmed early in the morning.
Continuity mistake: The mane on Lee's favorite horse, Traveler, changes from gray to white to gray.
Factual error: During the scene when Gen. Lee is talking to Moxley Sorrel in the cabin AFTER little round top, he calls Moxley "major", and moxley is wearing the insignia of a Confederate lieutenant-Colonel. By that date in the war he was a lieutenant-colonel, but a couple times Lee calls him major.
Answer: Mainly because his soldiers were tired after a long march and he didn't think they could make a direct assault on an elevated position (the hill was not quite as "bare" as the film suggests). There were also reports of Union reinforcements arriving to the area, and Ewell was afraid that his army might be exposed and overwhelmed if he attempted to take the hill without assistance (Ewell had requested, and been denied, reinforcements of his own).