Japanese has fewer phonemes than Canto or Mando, which always gave me the impression that it was easier to learn.

Maybe the added difficulty comes from needing to understand the grammar (particles) and the different forms of speech for different levels of politeness (after a few years, it now weirds me out when a white dude calls himself 'watashi').

Edit: Actually, if the methodology is "average time spent to learn" another possibility is that the average is slower because the barrier to learning the language is just low enough to attract a bunch of slow learners that wouldn't be interested in Chinese or Korean. (Reminds me of a study where decreasing page weight in KBs increased average page load time because users on the slowest connections were now able to load the page and skewed the average upward.)