Honestly not too sure. But now that you point it out, I wish Tiana had her hair down!

Tiana more or less lets her hair down, in the dream sequence - she lives during a time short hair was more common. Lottie goes for short hair, too.
Alan Menken originally was supposed to do the songs for Princess and the Frog, but when they decided on a New Orleans setting, they went with the fitting musician. I really don't think that Menken's style would have worked in this movie.
Flynn has "the smolder", so I suspect that he is used to charming ladies too (though he doesn't seem to be a womanizer just for fun), Rapunzel might be his first true love, but I doubt that she the first girl he had a fling with.
Rapunzel's many talents are barely adressed in her movie, while Tiana gets praised all the time for being such an outstanding cook.
Tiana turns into a frog because the movie is not based on the fairy tale but on a book based on the fairy tale - and in this book, the princess turns into a frog.
Really, what's the point of the question? Disney made two movies. One worked exceptionally well and one didn't. And in the marketing, they certainly favour Tiana so far, because she is 2D and fits better in the line-up.

I never really thought that Disney favors Rapunzel more than Tiana...
I think that Rapunzel is more popular than Tiana; at least in my experiences. All of my cousins and all of my friends kids (obviously their daughters!) went nuts when Tangled came out. They all wanted the dolls, and the dresses, etc. None of them were that crazy about PATF.

This is my main reason + DreamyGal's.

I think. Rapunzel is "special" for Disney & she's the 50th animated movie.

and I think 50 is a luck number
But when I was watching Alice in Wonderland (Tim Burton's) there was the trailer and I wanned' to see it so I've gone crazier than I though for the PatF

I guess just because she's more pupular, more people liked her movie, she was the first CGI princess, she's the last princess they are going to make for a while, and because she's just a better princess in general. (just my opinion)