Comp is made for the type of person that enjoys completing everything, getting platinum trophies or otherwise likes to 100% videogames. Conversely, jagex wants to add harder (group) bosses intended for highly skilled individuals and well-oiled teams. As you might have noticed, these 2 groups are not the same. By removing reaper and other hard pvm achievements from comp, comp is more accessible to the first type of player while jagex doesn't have to feel restricted by them when making extreme content for the second group of players.
I think this was more or less /u/JagexJack 's argument on stream (correct me if I'm misrepresenting him) and I think I more or less agree with it.