Because gun-welding violence is still extremely rare in China and even if occur, usually the Xinjiang region where automatic weapons are smuggled from neighbouring conflict zones, this thing is in response to the more likely knife-welding violence and the strict ROE Chinese law enforcement concerning firearms.
According to more pics posted, the mace are issued to those stationed at large public areas, 3-men standing post, one carry this scary thing, the other carry a shotgun and the other carry a fork-like pole that can pin the aggressor's torso safely. Think about it, unlike a metre-long pole where you might be able to grab onto the end to pin the policeman for that duration, you simply can't or won't want to try to do the same when against that! Unless, of course, you're so drugged up or drunk that you no longer capable of reasons.
Think of it like chainswords in "Warhammer 40000", just the presence of that thing alone speaks loudly of the intent and ability to inflict massive and messy pain. Works better than those tiny service pistol or that stupid revolver the Chinese police are issued with, unless of course you're carrying an AK, which calls for another level of game, and your opponent is now the SWAT team...
Like I said, in China size matters, and when something that big and with that many pointy bits, you got the intimidation factor covered in the ROE.
All of a sudden the sword baton from a while back seems awfully practical. This mace is ridiculous.
If they insist on going melee then I think they should issue armored gloves, arm guards, chest plates, single-handed hardened cattle prod "sword taser"s, and maybe riot bucklers.
Then there are pistols which should absolutely be standard issue for PAP. The revolver is ridiculous but does make sense for poorly trained regular cops who are just as likely to be targeted for it.
If their goal is to have the option to capture rather than kill then SMGs with rubber bullet and live round magazines would work better than fork and mace. Shotguns are good, can have the same options as the SMG as well as choice of buckshot or slug.
But obviously all this costs more money and training and probably isn't as fun or intimidating as playing with medieval or fantasy movie props.