Why socialism is not more criticized in humanist circles and literature?
There are many aspects that make socialism and Christianism on a par, and that makes socialism not fitted for reaching humanist values, which by itself should raise concern from humanists:
(1) Socialism was mainly developed out of either Christian religion or idealism, in both senses of the term, influencing the very socialist doctrine (see points (2) and (3)). This short history of socialism below is heavily based on the conference “Contribution française à la naissance du socialisme et du communisme” by Stéphanie Roza (a socialist history researcher):
_The first to propose a socialist theory was the idealist Greek philosopher Platon, in the Republic
_Then there were christian sects during the modern era, who considered that the advent of God on Earth was egalitarianism
_Then there was Thomas More idealistic book Utopia, which criticizes harshly private property
_Then there was the atheist Catholic priest Meslier who radically rejected social inequality
_Then there was Rousseau, who was much in favor of religion, and who was famously an idealist
_These ideas were brought through the Revolution by Babeuf (without mentioning the religious-like robespierrism of the Terror era),
_And received by Cabet, who first self-identified as “communist” and defended a form of “christian-communism”, the christianism-influenced mystic Saint-Simon (his book was called New Christianism), the idealist Fourier,
_And finally came Marx who heavily drawn upon the tradition described above and the German idealist Hegel and his mystical historicism, features which were only “scienticized” by materializing them
Since secular humanism, for it being naturalist and materialist, is antagonistic to both religion and idealism, that poses a problem.
(2) Christian religion and socialism all aim at the advent of a perfect world to come in the future (see point (1)). Since secular humanism cares of the here and now, and reject idealism, that poses a problem.
(3) As Christianism, socialism is mainly about virtue signalling, and the virtues in socialism (love, compassion, sharing, etc.) are, not coincidently, the same as Christian values (see point (1)). Since secular humanism is consequentialist, that poses a problem.
(4) Nor Christian religion neither socialism never ever improved significantly human condition (because both work from people giving resources to them, not from them being able to produce resources), but that never prevented people from still believing it and defending it (maybe because of point (3))