The entire Maryland Campaign was plagued by poor staff work by McClellan on down.

Even with an extra day the Army of the Potomac was still gathering from multiple points East and not 100% familiar with the ground. While McClellan retained his trademark focus on having a strong reserve, even if then he was loathe to commit it at any point. Here comprising Vth Corps and more particularly Sykes' Division of Regulars. So obviously a larger reserve leaves you with a smaller maneuver element.

But of course McClellan's plan hinged on a sweeping flank march, and there was a full Union Corps whose purpose was to demonstrate and pressure the Confederate right. That being Burnside's(well really Cox's) IXth Corps. A close relationship and active staff might have been paired with a bold enough commander to try a longer envelopment is certainly possible. But we can note several difficulties. 1 is of course the distance involved, as noted, the fords the Army intended to use to their North were a bit closer, and it seemed would let them take the Rebels in an open flank vs what portion of their lines could be seen to the East of town.

And there was the rapidly deteriorating relationship between McClellan and Burnside. Over the past few weeks McClellan had gone from counting Burnside one of his closest friends to lazy, unreliable, and potentially unfit for command. Some moments of indecisive command by Burnside were part of it, but muddled staff work by Army HQ, poor timing of messages, and wasted time in marches and counter marches made things turn sour. To the point that Hooker's 1st Corps was technically part of Burnside's Right Wing but was not actually under his command anymore. With Hooker instead being in essence the commander of the planned flanking movement.

But inadequate scouting work by both the AotP staff and not followed up on by IXth Corps meant that fords assumed to be beyond Burnside Bridge were not where they were expected, and that contributed to the delay in getting any sort of flanking pressure for the attacks on the bridge. By which time that corps had marched back and forth much of the day and McClellan was applying increasing pressure for Burnside to seemingly actually contribute to the battle.

Hey there,

Just to let you know, your question is fine, and we're letting it stand. However, you should be aware that questions framed as 'Why didn't X do Y' relatively often don't get an answer that meets our standards (in our experience as moderators). There are a few reasons for this. Firstly, it often can be difficult to prove the counterfactual: historians know much more about what happened than what might have happened. Secondly, 'why didn't X do Y' questions are sometimes phrased in an ahistorical way. It's worth remembering that people in the past couldn't see into the future, and they generally didn't have all the information we now have about their situations; things that look obvious now didn't necessarily look that way at the time.

If you end up not getting a response after a day or two, consider asking a new question focusing instead on why what happened did happen (rather than why what didn't happen didn't happen) - this kind of question is more likely to get a response in our experience. Hope this helps!