Because of the average ranges (USPSA) involved. I've used nothing but H&G 68's (
) (SWC) bullets. When I've shot Silhouette (TC 10"/.38 spe./.357 mag. Contender), I have found that "LRN" seem to be better at the longer distances (turkeys & rams).
Of course you must try each type at the different ranges that you will be shooting at.