It seems because, like many others, he disliked Darwinism. He preferred Lamarck. I'll say more about attitudes next time. Now I must quickly give some of the reasons why modern biologists are sceptical about Lamarckism. It isn't because they worship Darwin, it's mainly because of what we know about genes.
Most large organisms begin life as a fertilised egg, with a nucleus containing chromosomes.
The genes in the chromosomes are the material that's transmitted from one generation to the next. During development from the egg, the products of the genes interact with the environment. They're transmitted unchanged to the next generation. Suppose you developed thickened skin on your hands from hard manual work. That, of course, is a useful effect due to striving, you might say.