This was supposed to be the new spec era, with slim profile tyres, but COVID.
So they delayed it for a year. But there were concerns that teams would keep getting more and more grip from maximising the 2020 cars, putting a bigger load on the tyres than designed for(Pirelli didn't have a new spec lined up for this season on old spec). So these are essentially 2020 cars with a bit of downforce purposefully removed.