Twenty years ago, PNG wasn't really supported by any popular browsers of the day, but it showed its capabilities (especially compared to GIF) and was eventually incorporated. I'm assuming that you'll also say to me "You planned to move your entire business to a feature that has thus far never been shipped by any browser?"
In a more serious manner, JXL has features that both WebP and AVIF don't. Lossless images simply compress better with JXL than WebP (and if you in these circles you know to not bother with AVIF's lossless mode - even PNG beats AVIF in that arena). I know Google uses near-lossless images for their icons, but near-lossless wouldn't cut when it comes for medical diagnostics or meteorological uses. It has progressive loading, which WebP and AVIF don't really have. Speaking of near-lossless, WebP don't support 4:4:4 chroma subsampling while AVIF technically does but no encoder AFAIK focuses on it. Also, WebP only allows 16,384 pixels per side and AVIF only allows 65,536 pixels per side (which to be fair is the same as the original JPEG), while JXL allows 1,073,741,824 pixels per side (which is ludicrous, but considering that JPEG's 65,536 pixels per side was ludicrous at the time is good for future-proofing). There are also niche features which both WebP and AVIF don't have.
While I think that someone at Google was thinking that since AVIF beats JXL on compression at low bitrates there is no demand for JXL, that is a very narrow way of thinking. Not everyone is looking for bandwidth savings, some are looking that current image formats simply don't represent the things that they need (especially that most large companies have focused on "squeeze up the quality for lower bits").
In a more serious manner, JXL has features that both WebP and AVIF don't. Lossless images simply compress better with JXL than WebP (and if you in these circles you know to not bother with AVIF's lossless mode - even PNG beats AVIF in that arena). I know Google uses near-lossless images for their icons, but near-lossless wouldn't cut when it comes for medical diagnostics or meteorological uses. It has progressive loading, which WebP and AVIF don't really have. Speaking of near-lossless, WebP don't support 4:4:4 chroma subsampling while AVIF technically does but no encoder AFAIK focuses on it. Also, WebP only allows 16,384 pixels per side and AVIF only allows 65,536 pixels per side (which to be fair is the same as the original JPEG), while JXL allows 1,073,741,824 pixels per side (which is ludicrous, but considering that JPEG's 65,536 pixels per side was ludicrous at the time is good for future-proofing). There are also niche features which both WebP and AVIF don't have.
While I think that someone at Google was thinking that since AVIF beats JXL on compression at low bitrates there is no demand for JXL, that is a very narrow way of thinking. Not everyone is looking for bandwidth savings, some are looking that current image formats simply don't represent the things that they need (especially that most large companies have focused on "squeeze up the quality for lower bits").