Maybe it's because she's a bookworm. It just leads to more adorkable moments than with the others. Fluttershy of course will be cute because she already is. Rainbow Dash was a hyper kid and had a messy mane, so it makes her cute. Applejack just because. Pinkie Pie because it's Pinkie Pie. Rarity also looks adorable as a filly as apposed to grown up because then she's more pretty (which is the point).