In spite of the insistence of PPers that he is among the top 5 pacers ever, Steyn has not been able to become a household name like Wasim, McGrath, Ambrose, Warne, Muralitharan, Marshall, Holding, Lillee, Imran, Donald etc. etc. Even Brett Lee is more renowned, in spite of the fact that he is a much inferior bowler in Test cricket.
He has not been able to captivate the casual followers and they are generally unaware of his greatness. They obviously know that he is a good bowler, but they scoff at the idea of him getting bracketed with some of the names I stated.
I think the primary reason is the fact that he has had a forgettable Limited Overs career. The numbers are good but he has not produced any memorable spells in World Cups that can define his career.