watching the underdog finally scoring a win is more exciting than seeing the same guy winning for the thousandth time, who would've guessed

That is the thing, though: With Yugi, you just get used to it from the start. Meanwhile, Ash has never known that feeling. Of course people will cheer for the underdog here.

(Oh, and also, Yami Yugi's so-called "Heart of the cards" in the manga is ACTUALLY a power he has to just draw whatever card he needs at any moment, so, uh... that is kind of bullshit, you know. Ash at least earned his win with effort, even if it was a bit lackluster.)