Charlotte Flair was written out of WWE storylines as she is having elective surgery and will be out of action. Flair was being built up for a big program with Asuka. Thus, this is why Flair never lost during her time in NXT.
According to Dave Meltzer on the latest episode of Wrestling Observer Radio, the surgery and a potential new TV project are factors in Flair’s return to WWE TV. Flair is currently working on a TV deal, for some sort of reality TV project.
It was noted that Flair’s WWE return date is undetermined at this time.