The Criminal Code offence of fail to remain still requires mens rea to convict, so the Crown would have to prove that the accused either knew or reasonably ought to have know that there had been a collision. In this particular one, it's not like there was a big or even small "thud-like" crash with the rider that would make it obvious there had been contact.
If the rider was just lightly brushed and not knocked over or sent flying, then there would be virtually no noticeably impact felt by the driver of the car. The driver of the car just needs to maintain that he did not know there was a collision, that he felt no impact, that he though there was enough room to squeeze by without contact, then there is not much to prove any mens rea, and in turn it will be more difficult to convict for fail to remain.
A dangerous driving charge would probably have been much easier to support, but the cops can still come back and add that charge to the roster later.
