Because it’s just one library of many. It may be popular but it’s far from being the only choice. And it would also cause it to freeze at a particular version and make improvements much slower.
Plus there is little advantage anyway. It’s fairly small and you can set it to be cachable indefinably by the brower so it will only be downloaded once anyway – if you have a new version it will have a new filename, so there is no harm in making it never expire.