Wikipedia probably wants to support every user of the internet, which means even very old browsers. You can’t rely on relatively new CSS features when supporting browsers that are 10 years out of date.

The fact they’re still using JQuery, probably for similar compatibility reasons is good evidence of that.

Now there are ways to use polyfills that only load when necessary but just about everything is very difficult at wikipedias scale. We can’t solve their problems from an armchair.