I think it's because he was hated at the time for taking his investors' money and going to the South Pole rather than the North (basically Kickstarter rage), especially when he knew about the British expedition, then announcing it by sending the British expedition a sniggering telegram. Another reason, is that Scott and his four died carrying his joke letter back (the both one rubbing in and proving that Amundsen arrived first.) Lastly, I'd say that it's influenced by Amundsen taking almost the exact same route as Scott had previously announced.

At the time, I think he was seen as a heel that pretty effortlessly ended up as champion. The British with their bizarre obsession with manhauling definitely saw the usage of dogs as less manly. Also: Amundsen was not photogenic. More evidence that he was a cartoon villain.