I mean you could probably argue the individual members of Twice have had solo opportunities too if you look at each members' schedules over the past 8 years. I think the main difference is the talent. Taeyeon is just one of the best girl group vocalists of all time, so people wanted to hear her voice only and she debuted STRONG. Twice have never been known for their vocals, in fact they're often dragged for it, so the public would kinda have low expectations and not really care as much. I still think enough ONCEs would support any member going solo.
Twice's solo endeavours are not comparable to Taeyeon's at all, ignoring the fact that she was doing it when SNSD were at the peak of their powers in SK. Twice are finally taking small steps after.