Despite Tiana merchandise flying off the shelves, The Princess and the Frog was a (relative) failure for Disney, which caused them completely change their marketing strategy for Tangled. Their reasoning: the movies only appeal to little girls. Despite Rapunzel being an official Disney Princess, this was completely downplayed and her movie was made out to be a Shrek-like 3D animation romp. Since then, Tangled has done great at the box office, and Disney has said that they will not stop doing fairy tale movies.
Newer Than They Think: Despite the fact that Disney has been making movies featuring princesses since 1937, the Disney Princess franchise itself was only introduced in 2000; that makes it younger than other Disney franchises such as Baby Einstein and Toy Story.
Swan Lake, Bluebeard, Rumpelstiltskin, Aida, and The Love For Three Oranges were each going to be made into a film at some point by Disney (although the first was eventually made into a non-Disney animated film, The Swan Princess). Maybe their heroines would have been in this franchise.
Giselle from Enchanted was going to be given Disney Princess status as well, but Disney abandoned the idea when they realized they would have to secure life-long rights to the image of Amy Adams, Giselle's actress. Not to mention Aida got a Disney-supported stage musical.