There was a fundamental divergence of opinion between General Longstreet whose corps would have furnished the "missing support"and General Lee about how to wage that battle. It is a simplification but largely accurate to say that Longstreet was thinking tactically and Lee was thinking strategically, although Longstreet's tactics were to quite consciously exploit the strategic geographical map.
Lee wanted to bring the European powers into the war if possible even after the Emancipation Proclamation, he wanted to win a signal victory in the North which would break the morale of the North, and encourage the antiwar forces there. He knew that his army could not well subsist in Virginia but it have to get by quite apart from a logistic line of support by foraging in the North. Indeed the battle itself occurred because the Confederates had heard that there were some shoes to be had in Gettysburg. Lee felt that his window of opportunity to bring about a decisive engagement was limited by his lack of logistics, and his inferiority of force.
Lee, with some justice, felt that his army was invincible after Chancellorsville and the whole record of 1862. He knew that a long war meant inevitable defeat for the outnumbered and outgunned South. So he's famously said words to the effect, there is the enemy and there is where we will fight him. Lee sought a major victory to end the war before the weight of numbers and matériel simply overwhelmed his Army of Northern Virginia and the Confederacy.
Longstreet looked at the maps, one topographical of the area around Gettysburg and the other strategic of the theater of war. He saw from the former that the Union forces occupied the superior ground to the rebels' disadvantage and he sought to remedy that by reference to the larger map.
He argued that Lee should continue to maneuver to interposes army between the Army of the Potomac and a strategic city such as Washington, Baltimore, or Philadelphia, occupy high ground and compel general Meade to offer battle at is his disadvantage but which could not be declined because the Confederates had maneuvered into a strategic position which the Yankees must attack.
Advantaged by hindsight, I am inclined to agree with Longstreet but Lee was not so favored by second sight and determined to fight. Many have faulted Longstreet for being tardy. I think events have proved Longstreet evaluation of the terrain to be correct.
Another possible reason for the lack of support was the shortage of ammunition for the artillery which was supposed to soften the Yankee lines. A message was sent out from the artillery commander who said in effect, for God's sake if you're going to attack, attack now I am running out of ammunition.
Finally, there is the question whether there was any real lack of "support" considering all the factors, including the terrain, the Yankee artillery and strength.
If there had been a breakthrough, one wonders whether the Confederates had sufficient strength to capitalize on that local victory and decisively beat general Meade's whole army. It is been the tragic story of the Army of Northern Virginia that Lee could not capitalize on his brilliant victories because he simply did not have the resources to follow through. Without a decisive victory over the whole of the Yankee army, Gettysburg would not have altered the inevitable outcome of the war. After Gettysburg the Confederacy could not win the war but the union could in fact lose it at home in the North.
