why cant a firm earn supernormal profit in the long run?

In the long run, the number of firms is not fixed, firms can easily enter and exit the industry. The existence of supernormal profits in the long run will attract new firms to the industry and thereby increase the industry output.  Since, demand for the industry output is unchanged, the increase in supply of output will drive down the equilibrium price. Price will continue to fall till the supernormal profits are completely wiped-out. This is why we say that firms can't earn supernormal profits in the long run.

To read more on this topic refer to Chapter-4 (The Theory of The Firm Under Perfect Competition), Lesson- 4(Equilibrium and Perfect Competition – Short Run and Long Run), Topic- Long-Run Equilibrium.