I think it's because running takes a lot less equipment and time to prep for. What I mean by that is that to run, you only need a pair of running shoes and some decent shorts and you're good to go. Whereas, with a bike, you need a bike, a helmet, good tires, a good frame, everything. It ends up costing a lot more money than a good pair of shoes. That's probably why more runs are happening than bike rides - because the initial hurdle to become a runner is a lot smaller than becoming a biker.