why does no one in the world of ice and fire suspect that Jon Snow is Lyanna's son?

AFAIK, there are several theories about Jon Snow's parentage. But the likeliest to me is Lyanna and Rhaegar. If this were true, then how did Eddard Stark manage to keep this under wraps? Someone must have made the connection between Lyanna getting kidnapped and dying 9 months later and the sudden appearance of Jon snow. Why does no one suspect the obvious?