From the wikipedia page about the number of nuclear weapons we see that China only has 250 nukes, which puts it in the same league as France or the UK, and way under the 1000+ actives held by Russia and the US. That number has been unchanged for decades.
Yet, China is putting some major effort in deploying better weapon systems, has a somewhat aggressive foreign policy in the South China sea area, along with a number of deep held priorities, like Taiwan, that might involve the US against it.
Besides equipment that is still catching up to the West, Chinese troops lack experience, especially compared to US forces that have amassed considerable combat experience, albeit against technologically inferior foes, over the last 18 years.
This part is especially important, as nuclear weapons don't rely on combat-seasoned troops.
Last, but not least, the fledgling Anti Ballistic Missile defenses the US has deployed are not credible against a full-on Russian launch, but are more relevant against an opponent that has a limited number of nukes at hand. China is above that threshold, but not by much.
Have there been indications from China why they have not much strengthened their nuclear forces? They have the technology, funding and are not averse to competing in other areas. They're not much bound by law or treaties, being a recognized nuclear power, having their UN veto, and not being party to the various nuclear disarmament treaties signed between the US and Russia. It seems like it would go a long way to equalize their position wrt the US.
Note: while I don't rule out an official Chinese government explanation, nuclear armament is certainly a domain where countries use a lot of spin in describing what they are up to.
Also, I am mostly interested in the reasons for this restraint from a realpolitik Chinese military planning point of view, rather than other perspectives which, correctly, find nuclear reduction a laudable goal.