Plot hole: The whole plot of the first "Brother Bear" was that Kenai had to go to that mountain to become human. But at the end of this movie, the spirits were about to change Kenai back into a human in the middle of a cliff, which makes the entire first movie completely pointless.
Revealing mistake: Near the end, right before Nita goes after Kenai, she takes off her dress. If you look closely, when she takes it off, her necklace and earrings come off with the dress as if they were a part of the dress. Also, her hair is tied back when she's wearing the dress. But when she takes it off, her hair is loose and flowing.
Audio problem: Everybody knows that waterfalls are very loud. However, in the scene at Hakoni Falls, which is completely surrounded by waterfalls, it's silent as a tomb.
Continuity mistake: When Kenai and Nita reunite, there are two broken spear pieces on the ground. Halfway through the scene, one of the pieces disappears.
Continuity mistake: Near the end, Nita tears off her sleeve to use as a bandage for Kenai. At one point, she drops it and it's gone for the rest of the scene.
Continuity mistake: During the racoon scene, Nita completely disappears in wide shots.
Plot hole: Kenai getting bonked on the head with a pinecone at the end could never happen. When Nita walks over to Kenai and Koda, we see that the only trees are to Nita's right. The pinecone then falls on Kenai's head from ABOVE. They look up and see a tree above them. Cut to a wide shot, there are no trees around them.
Continuity mistake: When Nita's bag washes ashore after falling down the waterfall, it washes up very close to the water. But in the next shot, it has moved farther away from the water.
Continuity mistake: When Koda "scares" Nita with the fish, she drops the fish in the water. But later, the fish is back on the shore.
Continuity mistake: At the end, when Kenai and Nita's fiance are fighting on the cliff, the sun is just starting to set. But the instant Kenai falls off the cliff, it's completely dark.
Audio problem: When the ice cracks apart at Nita's wedding, a woman in the background says, "It must be a sign", but the movement of her mouth doesn't match the dialogue.
Continuity mistake: When Nita shows the amulet to Kenai, it's oval shaped. But for the rest of of the movie, it's circle shaped.
Continuity mistake: In the scene where Nita goes to see the sha-woman, at one point, the sha-woman drops her horn. In the next shot, the horn is gone.
Other mistake: When Kenai transformed into a bear, his clothes ripped right off his body. But when Nita transforms, nothing happens to her clothes. In fact, they seem to change with her.
Continuity mistake: When we first meet Nita, we see a wide shot of her village. At her wedding, the village is covered in snow even though earlier it was completely dry.
Continuity mistake: At the end, we see a wide shot of Kenai and Nita's wedding. A few seconds later, we see Nita walking down the aisle. Given where she was standing, we would've seen her in the wide shot. But we don't see her at all.
Chosen answer: Probably for just that reason - they were waiting for the right moment to change Kenai back.