Just like Omegaverse, I think it's because they tend to focus more on the MC's pov which is mostly the omega and guide. Hehehe, just my opinion.
You make a pretty good point!
As Caelophile said before. It's mainly because the main character tends to be a guide / omega . Another one there is Cakeverse , like fork exists too .
Now thinking about it bls mostly tends to have a bottom as their main character like there's very few top pov . Shouldn't bls be called bottomverse #－.－)
You're right, that's most likely the reason, And also maybe because omegas and guides are the main 'novelty' of that universe in general? I mean, omegas (men who can get pregnant) are the whole start and reason of omegaverse's existence and I think alphas were more of a consequence, while guideverse is basically just an evolution of the hunterverse genre where guides are the only addition. Basically, they're the center of their respective verse. "Esperverse" would just be confused with the classic hunterverse I think
Why is Guideverse called Guideverse? Espers exist too!!