I just ordered the 24GB one. I'm guessing it is not that common, since it says it's shipping all the way from China.
As for why 24GB? I'm a heavy multitasker and use Office 365.
Just using Office 365 Apps (Excel, Word, Teams, One Drive, Outlook), WhatsApp, Affinity Suite, Photos, dozens of Chrome/Safari tabs, Nova, Monday, Canva and Livechat makes my laptop unbearably slow at times with my current 8GB M1 MBA.
I suppose 16GB could have been enough, but I also occasionally use VM Ware, so 24GB made sense.