This is because the site is in the United States, where it violates no law and indeed is engaged in constitutionally protected expression.
The site in question, called SaSu, has already voluntarily blocked the UK. My client is not sure what more the UK requires of it.
Quote
John Milne MP 
@JohnMilneLD
I've joined cross-party colleagues in writing to Ofcom about a harmful suicide forum.
Bereaved families have shown immense courage in campaigning for the Online Safety Act. However, years later, after it's been passed, this site remains.
