Conditions in 19th-century London (no air-conditioning, crowded urban spaces, unhealthy summers) and the restrictions imposed by conventions (clothing that covered every inch of the body, and moreover had lots of layers even on hot days) made woods appealing in a way they perhaps aren't today. And the Victorians were not sun-worshippers intent on getting a suntan.
Result: paintings focusing on sunlight coming through trees and dappled shade beneath them.