Why couldn't it be in the 1940s? Were there any special events happening in 1980-1990 that relate to the story? I'm doing this for school and I couldn't find anything.
The first Harry Potter book is set in 1991. There's no deep thematic reason for that year. It's just the year when J.K. Rowling started writing the book. Harry Potter himself was born in 1980 but that was just so he'd be eleven in 1991.
But in 1990, my then boyfriend and I decided to move up to Manchester together. It was after a weekend's flat-hunting, when I was travelling back to London on my own on a crowded train, that the idea for Harry Potter simply fell into my head.
...well actually there was one significant thing that happened right at the start of 1991 (technically December 30th 1990). J.K. Rowling's mother died. And we do know how important the death of Harry's mother is to the story.