> For people who might not know, Rhapsody of Fire broke into in two bands, one keeping the original name and the other becoming Rhapsody.
Ehm, this is not what happened. In 2006 Rhapsody became Rhapsody of Fire to avoid some trademark dispute. Then they split around 2009, though the main chunk is still called Rhapsody of Fire whereas the spinoff is called Luca Turilli's Rhapsody.
The trademark dispute was with the Rhapsody streaming service - I don't think it was ever announced, but I suspect the streaming service paid them money to change their name since the band definitely came first.
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.
One thing I learned ~15 years ago was that Nightwish commissioned lyrics from lyricists (one such commissioned person told me himself I believe a year or two after Dark Passion Play was released - they never used what he wrote for them though) - maybe that helps give the polished impression for some.