I’m not really involved in javascript land so im parroting off of what i’ve heard for "why js over ts?"
it reduces file size since you no longer need to ship source maps
ctrl+clicking stuff will take you to the definition rather than an unhelpful type declaration
if you spot a bug in the library, you can edit the source directly than having to recompile/reimport
ts adds some unnecessary type “gymnastics” (can’t speak for what this means), when all they really want is intellisense thru jsdoc
So mainly: devs who don’t prefer strongly typed languages, and library devs who find typescript to be less transparent and more time consuming for new and old contributors than it’s worth