Trivia: Virtually all of the extras who play soldiers in the film were members of Civil War Re-enactor groups who not only freely donated their time but also supplied their own uniforms and equipment. This ensued that the film-makers did not have pay for their own extras and that they could stage the massive battle scenes without blowing the budget.
Trivia: During Pickett's charge, Armistead's brigade gets caught at a wooden fence. There is a close up of one of his aides rallying the men and getting shot. If you look closely you will see it's Ted Turner, who bankrolled the movie.
Trivia: Ted Turner's brief appearance is not the only notable cameo in the film. During the artillery bombardment on the Union positions prior to Pickett's charge, Union General Hancock is seen calmly riding through the barrage. A young officer begs him to come down and take cover. The officer is played by Ken Burns, the filmmaker who produced the acclaimed documentary series The Civil War.
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).