It was some sort of Disney princess redesign Disney decided on recently.  I know it applies to all the official Disney Princess merchandise and even the face characters in their amusement parks.  As to why, I can only make random guesses.  Probably part market research, part modernisation, part looking to regenerate/refresh the line-up to appeal to a new generation of little girls etc. etc… 

Personally I like some of them and some not.  Cinderella is probably my favourite revamp.  See here for before/after shot, she’s not massively different it’s just the new dress design works better as a real dress than the one copied directly from the animated film.  Come to think of it, that’s probably part of the reason too, all of the redesigns appear to have dresses that work better as real clothes so maybe it was all initiated from the parks, who knows?