the smoother the bottom the less chances there are or vorticies being formed which will cause different effects on the cars aerodynamics

you want the air flow to be as smooth as possible and any holes will generate different pressure spots, which, in turn, will lead to drag

yuo want the top of the car to be the one creating the drag as this equates to downforce and pushes the car down

creating downforce "underneath" the car will mean lift, which is bad