Continuity mistake: On the first day of the battle when Longstreet and Lee are talking, Longstreet's cigar is unraveling and in the next shot he has a new cigar.
Continuity mistake: At the beginning of the movie it is dawn of June 30th, but should be dawn of 29th. Harrison the scout completes his task and reports to Gen Longstreet. Longstreet confers with Lee and it is now evening. The next several scenes show a new dawn when Sorrell asks if he should wake the men, Buster wakes up Col Chamberlain and a bit later Gen Buford arrives in town with his cavalry and no fighting yet. This second dawn should be June 30th, not July 1st, the first day of the battle. Further that day ends with Buford drafting report for Gen Reynolds.
Continuity mistake: After Chamberlain's bayonette charge, he sees his aide wounded for a second time. The scene changes angles back and forth during their talking of how brave the boys were. An American flag is over the shoulder of the aide on close ups, but on a longer shot the flag is leaning against the tree.
Continuity mistake: During the sequence at Little Round Top, the same footage is used twice. The 20th Maine soldiers discharge a volley of musketry and there is a shot of a Confederate soldier being bodily flung backwards. A short time later, the Union defenders send down another volley and we see a close-up of another Confederate falling. Only it's the same actor, same footage, from a different camera about 35 feet down the Union line.
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.
Continuity mistake: At the beginning of the movie when the scout is talking with Longstreet, when the scout holds up his finger and says "you didn't know they were on the move out there", a long stream of troops is seen marching to the right. When the shot cuts to Longstreet then back, they have disappeared. (00:09:55)
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.
Continuity mistake: The mane on Lee's favorite horse, Traveler, changes from gray to white to gray.
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).