People are short sighted. Adding bicycle and pedestrian infrastructure will improve traffic in the long run, but the results will not happen right away.

Even people who actively want to bike can't use that infrastructure if there isn't enough of it for them to get where they need to go, and once there is enough of it it will take everyone else a while to start adopting it into their routines.

The way most American towns are layed out with all the residential areas, industrial areas, retail areas, etc segregated off into their own areas contributes to this because the average person has to travel much greater distances in their day to day activities, which is that much more infrastructure that must be added and also discourages people from taking non-motorized transport because of the sheer amount of effort and travel time. I live in a major city and the nearest grocery store is 4 miles away; it's a 1 hour round trip on my bike, and there's only bike infrastructure for a quarter of that.

Towns will add bike lanes to a few roads and when no one uses them (because no one has their house and their place of work/grocery store/etc on the same road) decide they aren't working and not add any more.