There is a reason for it. WebP was designed for a narrow-use: optimize for web. It doesn't support HDR, limited color gamut. e.g. All the devices that you use today are taking and viewing HDR pictures, or Print workflows, webp has no role to play in it.

This is not enough for companies to invest heavily in.

JXL is broad use/ universal. High bit depth, wide gamut, different color spaces — from HDR displays, to web, to print to medical photography and so on. It would help the device that your viewing this on, right now.

It's not like WebP where the only focus is to reduce file size. The fact that it does this also, and also recompresses jpegs lossless, is just an addon, not a singular reason.

If you get a chance, ignore all the file size noise, and look at JXL for what it is and how it's being adopted by different industries.