Because that was the basic plot of the movie. I don't think she would have developed, I think she would stay the same.

She and her mother had a huge fight before she turned Elinor into a bear, and I think the fighting would've gotten to a point where they were eventually forced to see eye to eye (convinced by Fergus, perhaps). They would have definitely both sat down and discussed how they felt and Elinor would have probably changed her mind about the marriage which Merida would be grateful for. It would've brought them closer anyway, in my opinion.

Without such a traumatic event happening like that, the two wouldn't have bonded most likely, which plays a big part in Meri's development.

Eventually... lol If not the bear thing then probably something else would have happened to sway her views..

She cared about changing her this is why she was always with her mother and they had the chance to have fun together. Otherwise, they would always argue.

Her mother would have kept forcing her to bend to her will, she would have kept resisting, and it all would have descended into a downward spiral.

Just by narrative convention she would have to change, otherwise it would just be two people arguing back and forth until something gave in. I personally think it would have been a better film if the bear thing was taken out and her kingdom actually went to war with the other clans (a real war, not that little hissy fit in the throne room between the clans) that way Merida would realize the gravity of her actions and either relent or find some way out of it. Elinor could perhaps see that marrying a bunch of teenagers off is no way to secure a kingdom (it's not even an old tradition since they mention that Fergus became king after the tribes came together to fight outside invaders). It would certainly make a more interesting film than "whoops! Me mum's a bear!"

the point was that the bear thing was the catalyst to change