Because Favors is more of a proven commodity and further along in his development than Embiid. Lower risk, and maybe a franchise would prefer to win sooner rather than later (Embiid is a project).
But if you're projected to get #7, there is also a chance you'll luck out in the lottery and get top #3 pick.
Also if your team is bad enough to be #7 worst in the NBA, I don't think that team is any where near ready to pay big $ for Favors.