in small signal analysis why only ac currents and voltages are used for calculation of resistance
As a result from small-signal analyses we get the gain of a stage and - if desired - the input and output impedances of the circuit. All these parameters depend on the small-signal voltages and currents only. This is because they are derived from linearizing the relevant (non-linear) characteristic curves around the selected bias point. 

However, the VALUES of the small-signal voltages and currents depend on the selected bias points on the non-linear DC characteristic curves. This is because they are identical to the SLOPE of the curves at the bias point.

Insofar, all DC voltages and currents are, of course, taken into account (but only indirectly, because they do not appear in the small-signal equivalent diagrams).