I guess it would be because Celestia hasn't really shown many flaws at this point, which makes her become viewed as a Mary-Sue kind of character. Personally, I'd love an episode that delves a little more into her personality, as Luna already got one.
Speaking about Luna, she's socially awkward, so that's a flaw of hers that makes her more relatable.
I do like Luna more, but that could change with a Celestia episode.