Why are curses limited to level specific enemies, what?

Is it not normal to expect your skill to work?

What do you mean?
Skill gems have something like effectivnes by levels, it goes for curses, exposures and other debuffs.
It will not be able to effect monsters above x level depend of the level of the skill.

I think 18lv(or 19?) is the first one that work on any level of the mobs to be effective