Because they can't be monitored on camera by an adverage human being. Their movements are to quick for that and that Yusuke didn't gain that kind of speed until after the dark tournament. Yusuke defeated guys much slower than Bomber at this point and wouldn't be able to keep up with his movements or wouldn't have an answer to his ability.
Originally posted by Classic NES
The counter for it is relatively simply. Assuming Yusuke is as good at partitioning his aura like Gon.
Yusuke isn't faster and he can't use his aura to defend against Bomber's attacks because his spiritual power is different from nen and he is unaware of Bomber's abilities to begin with. Gon had to actually train to defeat Bomber.