My guess on the reason why is two-fold. First, we've been telling people that Basic Auth is a terrible thing only stupid people would use for more than two decades now, and once a lesson like that gets hammered deep enough into the mass consciousness it tends to stay there even long after the underlying facts have changed. And second, using Basic Auth means giving up any possibility of styling/designing your login experience and relying instead on the browser's built-in dialogs, which tend to be clunky and ugly in the extreme.