Why was there no “deconfederitization” in the U.S. following the civil war, like there was “denazification” in Germany post ww2?

To some extent, there was; it just didn't last. It really depends on which phase of Reconstruction we are talking about when it comes to how best to handle a topic like "de-Confederization."

To begin with, even before the end of the war, there was a split in opinion about how best to handle the former Confederate territories and citizens. The most anti-slavery and anti-Confederate faction, the "Radical Republicans" tended to favor a Reconstruction policy that was longer, more punitive to ex-Confederates, and under the auspices of Congressional leadership. The Lincoln administration preferred a policy that was shorter, more focused on reconciliation, and largely under the auspices of the Presidency. These clashing views came to a head when Congress passed the Wade-Davis Bill, which stipulated as a condition for "re-admission" (ie a return to federal representation) that a majority of males in ex-Confederate States take an "Ironclad Oath" that they had never supported secession or the Confederacy. Clearly, this would have been an impossibility for the current generation throughout the South, so the subtext is that Reconstruction would be a generation-long process, and that the federal government would be in control of the South for a good while. Lincoln, by contrast, engineered a wartime Reconstruction policy of a "Ten Percent Plan" that would have allowed States to form a rump government if ten percent of the voting populace swore an oath of allegiance to the Union and foreswore the legality of secession. To circumvent Congress' plan, Lincoln "pocket vetoed" the bill, refusing to sign it before Congress went out of session, with hopes that cooler heads would prevail when Congress returned to session in 1865. This likely would have caused additional friction between Lincoln and the Radicals when Congress returned in fall 1865, but by that time, Lincoln was already dead, and Andrew Johnson was President.

Johnson, at first, spoke the Radicals' language, when he promised to "make treason odious" when it came to dealing with ex-Confederates, but like Lincoln, he preferred a policy directed by the executive rather than by Congress. Though Johnson had no love for planters, having grown up in the less prosperous eastern sections of Tennessee, he was also racist to the core, and disdained Radical efforts to expand civil rights for former slaves. Though Johnson accepted the 13th Amendment as a "punishment" for the Confederacy if nothing else, he never accepted subsequent civil rights bills, and opposed the 14th Amendment. As most people are aware, infighting between Johnson and the Radicals eventually led to his impeachment and near-removal, as Johnson had few political allies--as a Democrat and a Southerner, he was not trusted by Northern Republicans, and as a Unionist he was not trusted by many Democrats, either.

All through this period of friction between Johnson and the Radicals, the South was wracked by violence as ex-Confederates clashed with freedmen and pro-Reconstruction locals (so-called "scalawags") who often resented the old regimes (these included small farmers, urban craft workers, and merchants with prominent northern connections who were eager to see greater ties with other areas of the country). Race riots and electoral violence characterized much of the period between 1866 and 1868. With Grant's ascent to the Presidency, and Radical control of Congress after 1868, more sweeping legislation was on the table. In a few short years, Congress passed two Amendments, several civil rights bills, and most significantly the Reconstruction Acts, which put ex-Confederate States under military jurisdiction (though the extent of this was pretty minor, far from the "bayonet rule" claimed by later Lost Cause historians) and made enfranchisement of freedmen a condition of re-admission to normal relations with the Union.

A few Radicals wanted to go further. Thaddeus Stevens, probably the most prominent Radical in the House of Representatives, suggested that the entire Confederacy be reverted to the status of unorganized territory and carved into new States as Congress saw fit. Though this never really took hold outside of the Radical core, there are some seeds of "de-Confederization" as you would characterize it in the 14th Amendment. First, this Amendment clarified that States are obliged to extend federal protections to all citizens--this was a big deal at the time, because that was far from clear in traditional Constitutional law up to that point (States could, and did, enact censorship and sedition laws that are prima facie violations of the First Amendment, but that was not considered to apply to States prior to the 14th). Second, it explicitly disenfranchised ex-Confederates who had previously taken an oath to the US (eg military officers and elected officials), with the stipulation that Congress could remove these restrictions on an individual basis. Third, it invalidated all Confederate war debts, another major bone of contention from both individual and international creditors who had lent money to the Confederate government during the war.

At the same time, the Grant administration did take some aggressive action to root out white supremacist violence among ex-Confederates by deploying federal troops to suppress the Klan and similar groups (again, contrary to popular myth, the Klan was not a singular entity, and was only one ex-Confederate partisan group among many--much of the later iconography and romanticism regarding the Klan came from media like Birth of a Nation). Congress held numerous hearings to examine the roots of violence in the South, and over 1000 Klan members were prosecuted by Reconstruction governments. The Klan more or less ceased to exist from about 1872 to 1915 or so, when Birth of a Nation rekindled interest in the group. This was probably the height of Radical Reconstruction and attempts at de-Confederization. The Radicals hoped that political leadership would pass to commercial interests in the South and an educated elite of freedmen (always as minority partners, as plenty of Republicans still harbored white supremacist sentiments--the idea that most Republicans wanted to turn the South over to "black rule" was neither possible nor plausible in 1871), leaving the planter class on the ash heap of history. But this was not to be.

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