i know elsa is technically a queen, however, it's very weird to consider merida a disney princess and not elsa, considering the former is from a pixar movie.
Frozen was big enough to warrant its own line outside of the Disney Princess franchise, whereas the others were not. Don't get me wrong, the others were absolutely popular in their own right, but Frozen was one of the biggest box office hits for a film ever (and I think that both films are in the top 10 highest grossing animated films ever, though I'm unsure which is higher on the list).