[SEJeff]

Because jQuery was written to paper over limitations in ancient browsers that no longer exist, and modern (es6+) vanilla js is actually quite good. One of the primary reasons jQuery was invented is not relevant today. It’s still good software, but I’d say the same about the Perl lwp library yet you don’t see people using it.

[tambourine_man]

Cross browser compatibility was just one of the reasons for jQuery's success. But lots of other competing libraries offered it too at the time. That was the bare minimum a library had to do.

The reason jQuery was so successful, is still so widely used and was instrumental in fomenting HTML5 and killing Flash was its API. Nothing has come close to it in terseness, elegance and power.

Just skim this thread for examples.