Bob Weiss says Tiana is a strong character to focus the attraction on, and the connection between the film’s New Orleans setting, and the fact that Splash Mountain borders New Orleans Square at Disneyland, was also a factor.
In a recent interview with Disney fan club publication D23, Bob Weiss reiterated a previous statement from Disney Parks that the decision to retheme Splash Mountain pre-dates calls from fans to do exactly that.
However, it seems that the thought process that led Walt Disney Imagineering to settle on The Princess and the Frog for the new story was very similar to that which led the fans to suggest that same idea.