Multiple reasons:
1) Evidence is beginning to show that heat may prevent the spread of COVID
2) August is supposed to be a valley in number of cases
3) Giving up saturday TV slots in the fall, NFL will take them and potentially never give them back
4) This university makes alot of money on Spring football
5) Other conflicts for the university of having many sports starting all at once.
October was discussed as the latest month to start football (with condensed schedule).
As far as number 1 goes, I'd hate to see what S and SWFL would have looked like if it wasn't slowed in in heat.