I think it is because NA was never made to ever get caught up to KR dmg, There for they limit it all to top rank of CM, where u spend hours breaking scores, or you ask players in CN what glitches/bugs you can do to get extra points that most ppl wont know/do. This is another reason why NA has been at 20/20 HM scrolls for the past 1.5 years?(kr as 75 or more, TW was at like 45 last i seen) also why we are locked on 16 skill points (more if u get amulet to drop) (kr has like 25-30, even TW has way more than NA)
The devs want to limit what we can get, so when we can get it people are willing to spend money (new soul/pet psyches where the prime example of this)
so i guess to answer your question it is because that is how Devs want it to be in NA, and if you want them you will needed to get maxed geared players in NA that are good, get a hold of CN players and get the glitches/bugs. and out play the other ppl that do the same things for rank 1 CM.