American Civil War commanders had good reasons to order frontal assaults throughout the war, not least important of which was because they worked. For every Pickett's charge, you have a Gaines Mill; for every Fredericksburg, there's a Spotsylvania. Everyone knew frontal assaults weren't ideal; they'd all gone to West Point and studied under Dennis Hart Mahan, a relentless advocate for fortifications and defensive fighting, but in war, every combatant possesses an independent will. Just because you want to do something doesn't mean you can do it, because the enemy also is trying to do things to stop you and making you do things to stop them.

Technologically, the gap between a Civil War battlefield and a Napoleonic battlefield is practically nonexistent. Whatever the manufacturer touted as the range of their rifle muskets, infantrymen can still only carry about sixty rounds on their person, and combat almost always takes place at ranges under one hundred yards. And when you look at Napoleonic battles, the casualty data bears this out. In the American Civil War, bloody battles often cost a side twenty or thirty percent of its men. In the Napoleonic Wars, a side often lost twenty or thirty percent of their men. Same in the wars of Frederick the Great, the Wars of Louis XIV, all the way back to the days of the Roman Empire.

The rifle made almost no difference in the American Civil War, because a weapon is only as accurate as the soldier aiming it, and the men recieved very little marksmanship training before going off for battle. On the battlefield, a soldier can't be more accurate than his ability to see allows; within minutes, he would be practically blind and deaf from the crash of cannon and musketry, with thick clouds of opaque smoke all around. If the enemy can set upon you with bayonets lowered, you've got to get out of there.

Before the war, though, there had been some debate about drawing up new tactics to work with longer range rifled weapons (on the assumption that the troops would be able to use them with accuracy). One of the most prominent concepts under debate was called Zouave tactics, based on indigenous irregulars the French had fought in North Africa. Units fighting in the Zouave style would disperse under fire, each man getting plenty of space to himself. They would cross the battlefield in a series of individual jogging bounds, before going to ground, lying prone to fire. Orthodoxy held that troops had to stand up to reload a muzzle loading musket, but they had devised a means to reload while lying prone. After discharging his piece, the shooter would roll onto his back or rise to one knee to reload; in the former method, the soldier would brace the butt against his foot while he rammed the bullet in the chamber. There were some units that were labelled Zouaves and fought in this style, but it was impractical to train the entire million man army in this style.

Furthermore, I think the consensus in Civil War history is rather 'selective' in criticizing commanders' tactical methods. They might lambast Lee for ordering Pickett's Charge, but they also mock McClellan for not ordering hideously bloody frontal assaults against fortifications. He preferred to make fortifications of his own, then clear whatever was in front of him by hauling up the heavy siege guns and blasting them out of the way, at minimal cost to his army. And I think McClellan was right about it; if he had his way, and could entrench as he pleased, methodically marching and digging his way up the York-James Peninsula, there would be nothing the Confederates could do about it. But after his falling out with Lincoln, anything McClellan related became radioactive to the administration, so they ordered the Army of the Potomac to make advances along rain lines in the open spaces of Northern Virginia, which brought them into big, bloody battles with Lee's army.

Earl J. Hess has a lot of good books out about Civil War tactics, including

Field Armies and Fortifications

Civil War Infantry Tactics

Rifle Musket and the American Civil War: Myth and Reality

Paddy Griffith characterizes the American Civil War as essentially Napoleonic in his Battle Tactics of the American Civil War

Also check out Joe Glatthaar's General Lee's Army: From Victory to Collapse

Gary W. Gallagher has a book of essays on the Chancellorsville campaign, where Lee's army emerged victorious through brutal bayonet assaults on entrenched Federals

Ethan S. Rafuse examines McClellan's methods in McClellan's War: The Failure of Moderation in the Sectional Conflict