If sheetrock prices triple after a disaster, then what's to stop an enterprising man with a truck from driving outside of the disaster zone and picking up a load of sheetrock to sell for a profit? What's to stop 100 enterprising men from doing that? What would be the result? One hundred enterprising men would have money with which to rebuild, more people would have sheetrock and sheetrock prices would come down. Win-win-win.
If prices are fixed, what incentive would anybody have to truck more in? Result? Less profit, no additional income to enterprising men and a sheetrock shortage. Lose-lose-lose.