To be more accurate, it's motor vehicles and the people who drive them. The category is, "Things that make people quit cycling on the road."
There is no last word on this topic. It shows no sign of ever going away. Those who choose to pedal must now and forever deal with the challenges of sharing space with large, fast vehicles, mostly piloted by people with minimal training. And professionalism is no help: truck and bus drivers are notoriously hostile to pedalers. Professionalism may make matters worse, because those drivers are on a schedule and are earning their living by driving. The direct monetary connection reinforces their territoriality against not just cyclists but against all amateur road users.
The solution comes not with a single stroke but with a multifaceted response that has to include a lot of infrastructure changes along with behavior modification. Unfortunately, the system we have evolved developed very naturally along the path of least resistance. People were happy to let their communities be designed around motor vehicle flow. Almost no one questioned it. Forget whatever sinister conspiracies underlay specific things like the destruction of streetcars in favor of buses, and other sabotage of public transit. The proliferation of cheap automobiles relative to rising incomes in the mid and late 20th Century guaranteed that they would dominate our lives. The illusion of freedom was easier to sustain when the consequences, both economic and environmental, could be more easily masked.
We all understand the problem, but it seems as difficult to solve as gun violence. Both motorist dominance and gun violence breed fear, which can then be used to control people. In the case of cycling, fear serves to keep riders off the road.
People who used to ride tell me that they can't anymore. Maybe they quit completely. Maybe they switched to separated venues ranging from mountain bike trails to sedate paths. Most of them express their decision as a matter of maturity and wisdom rather than defeat and surrender.
People who haven't ridden on roads much or at all, who take up cycling or continue it in separated venues also assume the mantle of mature wisdom rather than regretful fear.
We all want to feel good about ourselves. Most of us, anyway. The problem is that the ones who have surrendered have surrendered completely. They've put it behind them and will not advocate for road cycling. I have not met a single quitter who said that they would take it up again if they noticed that conditions had improved. It falls to a shrinking group of experienced riders, augmented by younger people who are still in their riding phase, to keep a scrap of territory available to riders willing to face the existing reality and continue to promote proposals for its improvement.
The inexperience of those younger riders hampers their ability to understand the experience of cycling as the body ages. What was good enough for me in my twenties is out of reach to me in my sixties. It takes a bigger and bigger truck, going slower and slower, to get me to sprint it down. The degeneration has been gradual, but, because I have never stopped riding, I have been able to observe and document it. I guess I do all right for my age, but without the explosive power and grinding endurance I enjoyed from age 20 to about 50. And it shouldn't always be about exploring one's physical limits. Transportation and exploratory cycling should seldom be about exploring one's physical limits, or the limits of one's courage. It's okay for daily life to have a certain serenity.
To make this post self-contained, I have to acknowledge that motor vehicles have their uses. Time, distance, payload, and weather can all make a closed, motorized vehicle a better choice than something powered by human muscle. That has to factor into the system. When you need them, you need them. And what Edward Abbey called "motorized wheelchairs" can accommodate anyone who has decided that it's time to settle into their embrace.