Why is main reinforcement always kept below the distribution reinforcement?

First of all lets have a look at meaning of both ;

1. Primary reinforcement (Main Steel) refers to the steel which is employed to guarantee the resistance needed by the structure as a whole to support the design loads.

2. Secondary reinforcement(Distribution Steel), also known as distribution or thermal reinforcement, is employed for durability and aesthetic reasons, by providing enough localised resistance to limit cracking and resist stresses caused by effects such as temperature changes and shrinkage.

So the main reinforcement is provided to resist the load (tension). Now where is tensions is prominent in beam or slab of course at bottom. Thus more resistant to tension is achieved by providing the r/f at further bottom.

Also The main r/f is the one which resist the bending and takes the whole load transfer it to column. But if we provided the r/f vice versa(main at top and dist. At bottom) then the load taken by maim r/f will be eventually transferred to dist. r/f and as the dist.r/f isn't designed for that thus the structure will fail.