Page Previews, specifically Hovercards can be implemented in a very simple way with plain JS [^1] and is clearly an enhancement - everything works just fine if hovercards don't load.

But most features on Wikipedia aren't like that, and would be really hurt if you had to a JS bundle or otherwise deal with the usual bloated webapp nonsense. Wikipedia primarily needs to load quickly and be semantic with preferably simple infrastructure to help their budget.

[^1]: See Gwern.net for an example of an easy pure JS hovercard - https://www.gwern.net/DNB-FAQ