Well, she is the most important character after the protags and is Tamamo's foil so it's only fair they dedicate time to her.
Legit question, how is Koyan actually different than, for example, TamaCat.Koya would be more interesting if she wasn't handwaved into having the most lame excuse as to why she looks, talks, and acts like Tamamo despite not being Tamamo but sort of actually is?
She's almost the same as Tamamo in looks, actions and speech but has a different origin than the original.
In fact, Koyanskaya is even closer to Tamamo in personality than Cat.
The only difference is that she is not a tail that came directly from Tamamo, but how is that relevant? She's a separate character nonetheles.
And if the answer is that you didn't want Koyanskaya to be a tail anyway, then your expectations were misplaced to begin with.