> The simpler answer is probably that biking requires physical effort and exposure to elements
If this were the primary factor, the top cities for cycling would be places with mild climates and active-oriented population. That is not the reality as I see it.
> No doubt some combination of all these factors are at play
The primary factor is unambiguously the infrastructure. Among top cycling cities, the only common trait is good bike infrastructure.
> presuming that a materially greater portion of the population would start biking if we could only solve this one aspect of law or gear
No presumption needed, that is what literally happened in the Netherlands. They created good infrastructure, and went from basically zero cyclists in the 70s to the hefty majority they have today. The top cities for cycling (by ridership) in Europe are in northern climes with good cycling infrastructure. People in these cities are exposed to cold rain and snow.
see: https://copenhagenizeindex.eu/ , no correlation between climate and ridership.
Speaking to your broader point that people are getting lazier - I believe that humans are exceedingly adaptable above all else. If it is fast and convenient to get from A to B in a city by bike, people will ride bikes. That is what happened in the Netherlands and other top cycling cities.