that is because, in pachytene stage, the chromosomes appear as an independent unit or we can say that the packing of chromosomes is completed in this stage and then they are adjusted as near as possible( the homologous chromosomes) that's why the crossing over starts in pachytene stage of cell division. In proceeding stages, the distance becomes more and the chances of occurence of crossing over becomes less, that's why at the proceeding stages chances of crossing over are less but there is a attachment at a specific place formed in pachytene stage i.e the crossing over.
Jamil