I don’t quite know enough about Wang Mang to answer your question, but it does seem - at least from how you’ve posed it - that you’re under the impression that his efforts were successful. That was decidedly not the case. If you’re interested in the history of slavery in China, you can see my answer here.

/u/mikedash also provides a short answer to a similar question that does specifically discuss Wang Mang here.

I’m sure there’s more to say, so hopefully someone who knows more about the specific Wang Mang moment can still answer!

To provide some context, Wang Mang was two things at once: a very savvy politician, second to none in solidifying power at court and marginalizing his foes, and a passionate Confucian scholar who, after becoming emperor, used this platform to pursue a highly idealistic agenda of bringing about a Confucian utopia. His reforms can therefore be seen as attempts to shore up support for the new Xin dynasty and as efforts to remake China into an idealized Confucian state, based on precepts set forth by legendary lawgivers like the Duke of Zhou (a figure lauded in Confucian texts) and Confucian thinkers like Mencius. Wang Mang's reforms of 9 CE served to weaken the aristocratic political elite, which had accumulated both land and large numbers of enslaved individuals (see Rudi Thomsen, Ambition and Confucianism, for a fuller discussion of what Wang Mang was up to). As a member of this elite (albeit from sort of a nouveau riche family), who had himself usurped the throne thanks in part to civil and military authority inherited from powerful relatives, Wang Mang would have been rightly afraid that the gentry posed a serious threat to his rule. Anything that he could do to weaken the agency of these aristocrats would have been on the table. At the same time, there is evidence (see Thomsen) that Wang Mang adhered strictly to Confucian values in his personal life, so he may have harbored genuine convictions that simply matched well with his political strategies.

Wang Mang found historical and philosophical precedent for his 9 CE abolition of slavery in a line within The Book of Documents (Shangshu) discussing the Zhou state, which spoke against non-judicial slavery (see, again, Thomsen for more details). Although the reform did not take - much like the contemporaneous land redistribution reform that mirrored idealized land ownership discussions in the work of the Confucian thinker Mencius - it was enforced strictly before (like the land reform) it was lifted in 12 CE.

Note that when Wang Mang took power in 9 CE, he had a great deal of institutional power and public support, which would have afforded him enough legitimacy and credibility to at least attempt sweeping social reforms. As Shao-yun Yang indicates in an article published in the anthology Chang'an 26 BCE, Wang Mang rose to power at the end of an era in which the Western Han were increasingly viewed as headed toward a catastrophic collapse, and as Thomsen writes, the notion of a powerful and "pious" minister succeeding to the throne had legendary precedent. And through many years of savvy maneuvering, he had consolidated all "hard" state power under his family and inner circle.

Ultimately, though, Wang Mang lacked the governing discipline required to sustain sweeping reforms, failing to subjugate the aristocracy (whose resistance forced him to abandon his boldest early reforms) and became so caught up in his utopian designs (many of which were far less progressive than the reforms discussed here, and more technical in nature) that he lost sight of practical governance, leading to the domestic policy, military and internal security blunders that caused his violent overthrow.