The Files, why closed?

[Writerboy]

This just occured to me while watching Bill Waddell on the Shadow of the Ripper doc. Was there any particular reason why the Ripper files were closed for 100 years (albeit being opened in the early 70s)? Was this common practice for unsolved cases?

[Abberline43519]

The old Public Record Office would examine the records and keep them closed until a 100 years from the last date in the file.