The NFL really wants its players to get a COVID-19 vaccine. After a year of protocols, players missing time and hitting the reserve/COVID-19 list, and either no fans in stadiums or severely deflated numbers, the league wants things to return to normal as much as possible. That’s less out of the goodness of their heart than a desire to make as much money as possible, of course, but the intended end result is the same.
The league probably hoped they could just count on players to go get vaccines themselves, given that they’re widely available, but that process has been slow.