Why are slabs reinforced at the bottom?
It depends on the loading configuration of the concrete member. To determine where the steel reinforcement is needed you have to look at the bending moment diagram (BMD) which in turn can tell you how the member is going to deflect (deform).
In simple words the shape of the BMD is the shape or deformation that the loading forces the member to take.
In case of slabs the BMD and deflection are sagging downward, maximum positive moment and deflection at center and decrease to almost zero at supports. Which indicates compression at the top surface and the tension at the bottom surface maximum at center, which is where the flexular cracks are mostly to occur because concrete cracks in tension.
Ideally the steel reinforcement should take the shape of the bending moment diagram, because steel bars have high strength in tension and can also hold the cracks from opening.
This is why RC slabs are reinforced horizontally at the bottom, and you should typically leave a cover of around 5cm (~2″) depending on design, to protect the steel bars from corrosion because water can reach it through the flexular cracks at the bottom surface.
In other applications like prestressed concrete slabs the steel cables can take the exact shape of the BMD because it’s more effecient. but it is harder to excute and not neccesairy in normal reinforcement steel bars.