Is there a particular reason yaoi books are rated highly?

Well, except for some serious haters and trolls, the only people reading those books and rating them, will accept just about anything as long as it's bl. It's honestly just simple math; when a collection of people think something is the greatest thing ever, and they're the only ones proactively involving themselves with it, it would need to an absolutely horrendous pos for them to rate it badly. 

Because only BL readers read them, and anything BL are worthy of 5 stars for them. 

Because only hardcore BL/yaoi fans read them.

Its a very specific type of novel for certain individuals. Its not like isekai nor cultivation which is very broad in a manner. 

I see, I see. 

Only my speculation,
But since only yaoi fans reads them, them being yaoi is already a plus for them 

well said 

It's because they're good. 

From what I've gleaned:
It's usually the good ones that are TLed.It caters exclusively and is read chiefly by the target audience, resulting in higher mean rating.Some haters of the genre label it 1 star, leading to mass reactional countering (by the fans) with 5 stars. 

Low ratings usually  means the rater read it and disappointed they did not like it while those obviously yaoi/bl are avoided automatically so only who like the genre read it. 

Disregarding the yaoi stuff, the plot of those that I read is actually pretty good in its own way. Not that I'm super into yaoi or anything, I'm just reading cause I'm bored, plus the few yaoi novels that I read never disappoint me. 

Same I was thinking It was kind of a niche genre.