Because enough people from the 85th percentile will ignore it and I can give at least 2 good reasons:
1) Like most humans they suffer from the "won't happen to me" syndrome
2) They don't watch the same "news" or read the same rag that the service's PR department is advertising their blitz
Reminds me of the Park Lawn Gardiner ramps. They might as well build a couple of Timmies on each side, but people still get nailed there.
Then there's the third reason; as studies show, people will naturally drive at the design speed of a given roadway. That's where the whole 85th percentile concept comes from.