because he could score meaning dallas would have to score twice in 2mins with no timeouts. any other team and would have scored there, this is to rub it in to the dallas fans
What he did was smart, not rubbing it in. Had he scored, Dallas could've run the kickoff back for a touchdown, gotten the 2-point conversion, recovered an onside kick, and kicked a field goal to take it to overtime. Downing the ball meant that all Philadelphia had to do was kneel down until time expired.