Answer:
This question requires a personal answer. I will give you an answer that you can use as an example to modify it and make your own.
Step-by-step explanation:
This question corresponds to the poem "My mother saw a dancing bear" by Charles Causley.
We must begin by clarifying that the word bruin is used to name bears, especially in texts addressed to children.
That could be the first reason. The author may have wanted his poem to be addressed to an audience of children. If we read the poem with attention we can see that the poem has a happy tone and that there are even children in it.
Another option could be that the author wanted to avoid repeating the word "bear" so he used another meaning.