Seems easy to understand.
Some folks hold onto discredited ideas, long after they should just punt.
That doesn't even come close to explaining the phenomena. Why would leftists cling to Lamarkianism long after it had been discredited, especially when they had a highly creditable theory available, which you insist was congenitally favorable to their ideology?
Why did leftists argue that natural selection and random variation was inherently anti-revolutionary, and favorable to capitalist ideology? Why did even those leftists who considered darwinism compatible with leftist views go to great pains to refute such arguments (that is take them seriously)?
Obviously leftist did (and some still do, e.g. Jeremy Rifkin, many "new age" anti-Darwinists, etc) see problems with Darwinism, despite your insistence that they should have embraced it as ideologically favorable.