A Joker/Harley Film Would Have Done Much Better Box Office
The sad truth is, as bad as you wanna claim Jared Leto's Joker was, a movie with these two as a pair the whole entire time would have almost certainly done better than a film with just Harley on her own. And since we didn't really see much of Leto's Joker in Suicide Squad to begin with, most movie goers would have probably given him another chance to impress them. They could have still killed his character sometime before the end.
Plus they could have waited an extra week and released it on Valentine's Day. A film with these two as a couple being released on Valentine's Day would have been cool marketing.