I've always thought it's a combination of the genre being niche and being founded on a set of lyrical themes that are restrictive - and maybe particularly challenging -, and also, the fact that a lot of these bands aren't from English-speaking countries. The bands from anglo countries like Kamelot tend to write acceptable lyrics within the power metal context (Manowar lyrics are hilarious at times and arguably well-written, but they are not power metal IMO). There are some noteable exceptions to non-anglo bands with polished lyrics, like perhaps Nightwish or Blind Guardian, but a common occurrence otherwise is to find shitty English, which at least for me is an instant turnoff.