The general theory is if there are very few bikes on the road, drivers are not looking out for them. It's a clear and pretty well accepted fact that drivers get in the habit of only looking for things that they are expecting to see (that they see commonly) and miss unusual things (nobody needs to repost the basketball gorilla video, everyone's seen it). Also drivers don't know what to do around them and become unpredictable.

When cyclists are common, drivers are looking out for them and a common consensus about how to deal with them arises and drivers become predictable again.