Revealing mistake: In one of Pickett's charge scene the bayonets are obviously rubber.
Revealing mistake: There are white wristwatch marks on the tan arms of the Confederate soldiers as they reach for Lee as he rides among the troops.
Revealing mistake: A jet con trail can be seen just before the fight at Little Round Top, when Chamberlain and Tom are talking.
Revealing mistake: In the battle of Little Round Top, as the Confederate soldiers are backing down the hill there is one man walking backwards and loading his gun. Behind him another soldier aims his weapon and fires, shooting the first man in the side of the head. The man then ducks and waits for the smoke from the shot to clear and continues down hill.
Revealing mistake: Just before Pickett's charge, there are a number of shots of drummer boys playing the drums. Their drums have plastic drum heads with the "Remo" brand name on them.
Revealing mistake: There is a scene in which Union soldiers are standing on Little Round Top. You can see the General Warren statue, which is partially hidden by shrubbery.
Revealing mistake: In several scenes where the cannons are firing, they either don't recoil or recoil very little. In real life, they would have a bigger recoil.
Revealing mistake: In the scene where Chamberlain is walking with his brother just after Little Round Top and before they get to the safe part of the battlefield, there is a jet con trail in the sky.
Revealing mistake: When Buford's cavalry is first riding into Gettysburg, he brings his men alongside a fence. On the other side of the fence, there is a trail with tire tracks from a car or truck. They can't be from wagon wheels because they are about a foot thick each too well formed.
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.
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).