These had to withstand rough condition, being flexed by the wind loads they supported, attacked by sea and bad weather, and suffering from UV degradation from long-term exposure to sunlight.
Without elasticity, the varnish would soon crack, allowing water to penetrate the wood beneath.
Prior to the development of modern polymer chemistry, varnish production was rudimentary.
In modern times, "spar varnish" has become a genericised term in North America for any outdoor wood finish.
Owing to modern varnish materials, their weather and UV resistance is likely to be good, but the original requirement for flexibility has largely been forgotten.