I think it would make more sense to support the existing implementation than to start yet another one.
As for why jxl-oxide can't be used yet — it just isn't mature enough yet. They're still finding unimplemented or incompatible features, and have some unoptimized code.
JPEG XL is big and complex – it is designed to have every feature of every competing codec (from vector graphics to video), and beat them all on compression in every case, so it's a half a dozen of different codecs in a trench coat.
As for why jxl-oxide can't be used yet — it just isn't mature enough yet. They're still finding unimplemented or incompatible features, and have some unoptimized code.
JPEG XL is big and complex – it is designed to have every feature of every competing codec (from vector graphics to video), and beat them all on compression in every case, so it's a half a dozen of different codecs in a trench coat.