My god did I laugh at this, way too much.
I don't get where you are getting this notion. Rarity had more character-centric episodes than anyone else did in season one, and even in episodes where she was part of an ensemble cast (Best Night Ever) she still got more of the spotlight than everyone else in regards to character development because they tackled aspects of her personality which hadn't been covered before.
And constantly playing a support role in basically the same capacity each time (like Applejack constantly does) does not count as character exploration. That isn't to say that Sisterhooves and (to a lesser extent) Cutie Pox didn't go above and beyond doing so, but that still isn't the same thing as a character-centric episode.
Rarity has played out more supporting roles due to the stereotype she represents, I know that supporting roles doesn't mean character development but It should be said that the episodes like Fall Weather Friends do provide more insight into each character, while episodes like Look Before You Sleep only showcase Rarity as an stereotype rather than providing actual character development and character insight.
Also, lets give pleasure to our eyes shall we:
And for the people who thought Rarity could not get more Saucy:
I know what I want for Christmas.
(*Goes back to the exquisite AC04 soundtrack*)