I’m not sure I understand the title - AW does not use Qi but it is a proprietary solution, with the magnet and such.
I can only guess that was the best that Apple could come up with during development which goes back to probably 2014 timeframe when Qi was not that widespread yet…
Who knows what the future holds but given apples move to MagSafe for the phone, not sure we’ll see Qi for the watch.
edit:
Given the curvy nature of the back of the AW, that seems the most plausible reasons why no Qi to me