Why did nobody come forward whilst he was alive?
Surely it takes about a second to work that out?
The people who are now making the allegations would have run the risk of either being ignored because Jimmy Saville was a national institution, or exposing themselves to the risk of a libel action, had he chosen to take them on.
The fact that there are several people, and that their stories are much the same, adds weight to the idea that there may be something to them. It's possible, indeed likely, that others will come forward, now that the matter is in the public domain.
That remains to be seen. In the meantime, journalists will make the most of what they can discover, secure in the knowledge that you cannot libel a dead person.