In The Giver why is nobody allowed to be different?
Let us be careful here. What precisely do you mean by being different? It is clear that, physically at least, the citizens of the world where Jonas lives are allowed to be different. However, what is more disturbing is the way that conformity is created in terms of the strict rules that must be obeyed and the way that this community has bred out of its life concepts such as love, family and warmth, creating a kind of stale and artificial life, that some might argue is not life at all.
However, it is clear that this was done to avoid and eradicate the less pleasant aspects of human life, such as pain, suffering and disorder. Certainly, we must understand that this is a definite bonus of the world that Jonas lives in. However, at the same time, it is clear that there is something missing, as Jonas comes to discover when the Giver begins to share memories with Jonas of what things were like in the olden days. Note what Jonas says after experiencing a memory of love and family:
I liked the feeling of love... I wish we still had that... Of course... I do understand that it wouldn't work very well. And that it's much better to be organised the way we are now. I can see that it was a dangerous way to live.
Even though Jonas is not able to fully articulate why living with love was a dangerous way to live, it is true that when we love somebody we become vulnerable. Also, love is not a predictable emotion. It causes us to do strange, irrational things at times. Thus it is that the eradication of love helps maintain predictable humans, who might be argued to be less than humans and more robots.