To answer your question: Disney doesn’t always do the best decision making. Journey Into Imagination. The original California Adventure. Leaving a bunch of pads open at EPCOT for 40 years. Taking out any evening light parade at MK.
But your point was wondering "why Tiana"? And I answered, "Because Disney has always been promoting her all along." As well as the other members of The Disney Princesses (c). The board doesn't lack continual calls for a Tiana restaurant somewhere on WDW property.
Don't know why you would think the promotion of Tiana and Facilier, already both successfully promoted, is part of a series of bad decisions.