Character mistake: When Buster gets hit in the arm the first time, he is busy tying the kerchief below the wound. (02:21:05)
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.
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).