Why are Frozen's Elsa and Anna excluded from the Disney Princesses franchise?

There is no official word from Disney, but I can say these two things for facts:

• Elsa is a Queen. So, making her a Princess would be almost impossible.
• Elsa and Anna represent 25% of all Disney merchandising sales. Twenty five percent. Keeping them separate from all other Disney Princesses is probably the best move financially.

Being a Disney Princess is not just a matter of being a princess from a Disney property, it's a question of marketing. The collection of Disney Princesses is used together in other materials, things like direct-to-video releases, books, games, etc. There's an article on a Disney Wikia that has some more details, but that's the gist of it.

Frozen was such a success, that Anna and Elsa got their own marketing segment: Disney Frozen. At some point, the popularity of the sisters may go down and one or both may get rolled into the "Princess" marketing machine.

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.