But, why can't leak test be carried out with a particle counters ?

You can, but the basic function of a particle counter is to count and size particle, usually by sampling at a rather slow rate of 0.1 cfm with LAF, isokinetic sampling would be mandatory. scanning a filter seal in this manner would be a slow process involving great operator skill, and what is more important the reading obtained would have no relation to the established level for a significant.