Why was the purchase of Alaska so controversial?

As vice president serving under then-President Abraham Lincoln, Andrew Johnson was already a controversial figure, the lone Southern senator to choose to remain in Congress rather than resign when the state he represented, Tennessee, seceded from the Union. Ascending to the presidency following Lincoln’s assassination, Johnson remained a controversial figure for his support of racist policies in the American South, policies codified in what became known as “Jim Crow” laws, and for his opposition to the Fourteenth Amendment to the Constitution, which assured citizenship to those born or naturalized in the United States. Johnson would later be impeached as a result of his protracted confrontations with the Republican-led Congress over Executive authorities, especially regarding the handling of Cabinet-level personnel decisions—an issue that led to his impeachment by the House of Representatives (followed by his acquittal by the Senate by one-vote margin).
So, what does any of this have to do with the purchase of Alaska? Plenty. President Johnson’s secretary of State, William Seward, successfully negotiated the purchase of a vast tract of land from the Russian government. That tract of land, Alaska, was viewed by many Americans as a worthless and inhospitable region that, while certainly increasing the aggregate size of the United States, contributed nothing to the country’s overall development. The combination of Johnson’s unpopularity among many in Washington, D.C. and across the North and questionable judgments on the part of many of those same people regarding Alaska’s potential led to wide-spread criticism of Seward and, by extension, Johnson for the region’s purchase.
https://www.archives.gov/publications/prologue/1994/winter/alaska-check

https://www.history.com/news/why-the-purchase-of-alaska-was-far-from-folly

https://www.whitehouse.gov/about-the-white-house/presidents/andrew-johnson/

William Seward, Andrew Johnson's Secretary of State, purchased Alaska rather cheaply from Russia in 1867. Many thought that Seward, a holdover from the Lincoln administration, made a poor business deal, as few Americans knew anything about Alaska. They thought the land was barren and called it "Seward's Folly" and "Seward's Icebox." The land was not contiguous to the rest of the United States unlike other land acquisitions, and there was little chance that Americans would be willing to move there in 1867. There would also be some controversy with Britain about where British Columbia began and Alaska ended—this border would not be fully settled until the end of the nineteenth century. Seward realized that the ports of Alaska extended far down the west coast of Canada and these ports would be valuable to America's commercial goals in the Pacific. Seward also wanted to gain the rich timber and fishing rights in Alaska. Seward would not live long enough to see Alaska's true potential, as gold and oil would make the territory quite rich.