They are doing it this way because if this card was released with the other Sky Legends cards, then the legality would reset and Lillie would remain in the format for *at least* another year.
I don't think that Lillie was removed from Unified Minds/Hidden Fates just to be a Regionals promo. But rather, having it as a Regionals promo allows players to get an alt art that otherwise wasn't going to be released at all.
Also, these days, the promo is given out in various ways beyond playing in the main event. There are often side events where participants can get one (or a whole playset). Plus, you can trade for one. There are multiple ways of getting this promo at the event besides actually playing in it.