It is because in thee books, all the characters, except Newt, have american accents. It’s pretty simple actually. Newt was the only one with the accent. So his actor got to keep the accent, yet still change it slightly to match his character. Since the books were a HUGE sensation, the movies had to be as close to the books as possible. Especially the characters. One mess up with the Characters could ruin the whole plot ruining the whole story. You understand now?
I am a HUGE fan of the books so I pay attention to detail so yes I do know about all of this.