Question

why do acids not show acidic behaviour in the absences of water.

Answer 1

because in the absence of water can't found H+ ions .
And acid show it's property only in the presence of H+ ions.

Answer 2

acid do not show acidic behaviour in absence of water b/c acids like HCl do not produce h+ ions in the absence of water which is responsible for acidic behaviour.
hope it helps you