the thing is that dianas lore makes it clear that horros=void creatures,since she gets attacked by horrors who turn purple when they enter runeterras atmosphere and she closes the gate that connects to their home
And the void is definetly too big of a threat for noxus,it is a whole dimension of otherworldly terrors,that seems to threaten even other planets.
I mean in rek'sais lore it basically ends up with rek'sai being too much of a problem for the noxian.
And again,the question is:
Where would be the big use for noxus?
Conquering Targon doesn't get them much since targon is still a hazardous land.
diana could guide them,but if there is no guarantee for them that diana really knows targon that good and no reason for them to trust her...