My answer is in line with the previous. I just wanted to mention that
SuperCarlinBrothers have researched and made a video about this particular issue but in short, Frozen is just a powerhouse of a franchise on its own without the need for them to be Princesses.
There is a great chart at the 3:45 mark which shows that Frozen would account for 25% of the total ticket sales if added to the combined total of the other films. Most other princesses come in at around 8% - 15% each.
At the time they made that video, Frozen's merchandising sales were the 3rd largest behind Star Wars and Cars. They simply outclass the other princesses in terms of their value.