Why did Pixar and Disney feel it was necessary to change the direction of the story?
They felt it was necessary to change the direction to help improve the film, the original was to edgy and boring. Edge is used to describe it as sharp, snappy, rude and for adults. There were both advantages and disadvantages of this decision to change the direction of the story.
Some of the advantages are that they were able to improve the film and make it much more entertaining, another advantage is that they were able to come up with their own story and they had the power to choose what to make which allowed them to use their imagination much more.
However, there were a lot of disadvantages because of the decision for example, they only had a limited amount of time to come up with their own story and characters which meant they had to work all day every day to meet the deadline. They literally drew hundreds of pictures and concept art in a day which was really hard work.
Why did the artists create so many variations of the design before deciding on their final concept?
There are many reasons why the artists created so many variations of design for each character/object, as I mentioned earlier in this post, artists literally drew hundreds of pictures every day, but there are many good reasons for this. One reason is that you can get the perfect character design for the character and thats it fits is personality and how it sound etc. Another reason is so they can see what other people like the best and which one fits the desired character.