This is a conceptual question that you can put numbers to, to understand what’s going on.

Uplift is caused due to the wind “overturning” the building. Dead and live loads act downward (gravity loads) so they wouldn’t cause uplift in the frame stated in the problem. So to get the resulting uplift(tension) at footing A. You multiply the wind load times the height of the column and divide by the moment arm (distance between footing a and footing b). Now that you have your tension due to wind you subtract the gravity loads (dead + live) as they are all acting in the “y” direction, but tension is in the +y(up) and dead and live are in the -y (down). So to get the maximum uplift you have to look at the applicable load combinations that include dead live and wind. Thus the largest tension will come from looking at only the dead and wind load combinations as you’re only subtracting the dead load from the wind tension component, rather than subtracting dead and live loads from the tension.

I hope that helps