Because that's what The Sun (which also seems to have been chatting with Sussex spokespeople) implies:
"The force declined to answer a question from The Sun on whether they had forwarded any information from the letter to legal agencies abroad."
Several things could be going on here: 1) the Met examined the letter but didn't consider it credible; 2) the Met messed up and didn't pass the letter on due to error or negligence; 3) the Met did pass the letter on to the correct jurisdiction and the police there investigated and didn't find any basis to the allegations; 4) the police abroad didn't investigate the letter due to error or negligence.