What Could It Mean?
The San Francisco police had the letter in their possession for five years before releasing it to the public. They knew exactly what they were doing by hiding it away. They simply didn't want inquisitive people to find out about it within a reasonable timeframe...
Misusing Their Time
The US Marshals Service sent the letter into the FBI's laboratory for forensic handwriting analysis roughly two years after they received the letter. From what it seems, there's either something that the government is withholding or, they purposefully wanted to let the letter get old...