Smithman could simply be another innocent dad carrying his child home. For all anyone knows, he (or anyone else at home) could have been asked by the GNR when they did their house checks right at the beginning if he'd seen anything suspicious that evening. The answer would be no if he hadn't seen anyone else carrying a child or behaving suspiciously.
Tannerman was walking in a different direction and press allegations against Murat might have made him think they'd got their man and were about to charge him at any moment. Then there was a spate of Smithman articles, but the press seemed to have decided that that may have been Murat (cf Martin Smith allegedly denying that it was him), then there were all the allegations that it was Gerry and the McCanns were going to be charged at any moment. So there's still no reason to come forward... and the fear of being dragged into a media circus might be another. Or he may have simply moved away since then and not followed the saga.
Even today, depending on how the press present the police interest in the e-fits, it may sound as if he might suddenly be treated as a suspect (by the media, if not by the police), or be in trouble if he or anyone else associated with him was in an irregular situation at the time for some reason.
And then there's Amaral still plugging the Smithman-was-Gerry show on CMTV.
All in all, not very inviting to come forward, is it?