It’s because socialism is simple and earnest whereas capitalism is complex and shot through with irony. It’s easy to wrap moral language around socialsim but harder to do with capitalism.
It’s because capitalism is the natural state of man - that is, something that he produces goes to benefit him, while at the same time benefitting the community around him, and he defends his territory or property and uses it in the production of his goods. As social organization moved away from its tribal form and, with the advent of currency and a money based system, became more individually controllable and also more flexible (making capital more easily transferable and resulting in the rise of a moneyed class rather than merely an hereditary property owning class), it did so naturally and with little pre-planning. Property rights are based on the fact that ownership is natural to man. But because it’s a natural and therefore somewhat amorphous, unstructured thing, it’s harder to enunciate.
Socialism, on the other hand, is an artificial system that must be developed by an originator (Marx, for example) and imposed by force, since it is unnatural. However, precisely because it’s artificial, it’s easier to enunciate and, while it has never worked in practice, it’s easier to defend because one can always claim that it’s not being implemented properly.
The only people who benefit from socialism are those in charge of enforcing it, that is, those in government. This is something pro-capitalist thinkers need to point out unceasingly. Socialism is repression and never benefits the poor, even when it’s “soft” socialism such as in England, where socialism has created a huge, festering, going -nowhere class of the native-born poor, who have enough to live on and miserable government health care, but absolutely no prospects. Still, the government and everybody who is associated with it thrives. And in a more harsh socialist system, everyone who is not in the government or favored by it is absolutely crushed into the ground and deprived of freedom and even life. That’s a pretty good argument against it.
I think the main issue is that the mechanisms that motivate and animate capitalism are not immediately evident to people. The "Invisible Hand" of supply and demand, of capitalist water finding its own level is so foreign to liberals because it cannot be seen unless you understand it, then it becomes nearly obvious.
Liberals, however, can see the workings of man in a socialist-leaning environment, which is God to them.
They see people being elected, setting up meetings at 9:00 AM in government buildings with oddly named conference rooms where there are agendas and minutes and serious things to be discussed. To liberals, it is solid, just, and wise, the effort at making decisions. What liberals don't get, though, is...the decisions are more often wrong than they are right. But because they are made by tangible men, they are de facto superior to any kind of invisible mechanism (such as supply and demand)
Individual men making decisions about how to allocate scarce resources with alternative uses are not match for the combined intellect and experience of millions of men and women whose "demand" with money drives the "supply".
To liberals, depending on that is like worshiping God, putting your eggs in a basket you cannot see!
And to the people who depend on other men to tell them what to do because they can't think for themselves, this is an easy proposition for liberals. Capitalism is easy to demonize for them, to sabotage it. And the sheeple take their cues.