It seams other than 
no other champion is concerned by the void. Is an invasion from another dimension by creatures intent on consuming any and all life they come across not enough to warrant at least minor concern? We now have 6 pro-void champions (
, 
, 
, 
, 
, 
), and so far only a single champion that even seams to care.
Kass is biased because he got rekt by the void and will never be the same
Because most people don't believe in it. If a Darth Vader-wannabe walked up to me and started to ramble about an inter dimensional invasion by a bunch of purple bugs, I would run the fuck away instead of hearing what he had to say.
Because if the void invaded we would just get behind 
Kassadin is the Mr. Crocker of the League universe.
"There's only one explanation: Malzahar must have ignored my daughter's multiple birthday invitations, thus ruining her birthday. Feeling guilty, he impulsively loans her his VOIDLINGS! Finally, I have proof!"
"Uh oh."
"SKREEEEEEEEEEEEE!"
"YOU TELL HIM, HONEY. QUICK, MALZ, MAKE A WISH SO I CAN DEVOUR THE EVIDENCE."
"Mr. Kassadin, what do you think you're doing?"
"Principle Kayle! I've been looking for you! I have proof-"
explodes into a series of spasms that involves him warping in between dimensions
"THAT VOIDLINGS EXIST!"
"Uh huh. Sure."
Well this actually makes more sense since the retcon of the IoW,because now there is barely any evidence that the void exists,the voidborn aren't famous champions anymore.
is simply a big monster for the people of shurima and is also just a very strong monster for rengar.
And I think that rarely leaves survivors(or he might be too new to be considered a threat),while has a cult who follows him(so yeah he is teaching the people to join the void instead of trying to fight it).
For most people is simply a dude who walks around in a weird suit and talks about the apocalypse,they simply don't believe him,because they haven't seen any void monsters or only very few.
However it was said that he did create an order to fight against the void,but we have yet to see any of his people.
Because the evolving relationship between Katarina and Garen and the repercussions it has on the rivalry between Demacia and Noxus is FAR MORE IMPORTANT! >:0
Essentially because Riot didn't do a great deal to develop the Void while the world was still active. Kassadin and Malzahar had their little brawl back in 2011 but since then there's been nothing done to properly build the rising threat of the Void other than a couple of new creatures arriving with no real interest in anything beyond their own personal agendas. To me, the Shadow Isles feels more like it's in the Void's spot now as the looming ineffable threat that is pushing further and further with each incursion.
I have a theory that one of the reasons Viktor's targetting Void creatures for his Battlecast creations is not only to use their already formidable power to further his own goals, but also to eliminate the potential threat they pose to the world. Despite his... slightly aggressive actions, the Glorious Evolution's primary goal is the betterment of humanity (and yordles too, I guess) through the augmentation of their weak flesh-based bodies.
A Void invasion poses a great danger to all of Valoran's current inhabitants. Viktor is a genius, and knows this. By converting Void creatures to Battlecast machines, he is eliminating the small foothold the Void has gained in Valoran. As the BC-Vel'Koz preview showed, Viktor is gaining knowledge of the Void from each addition to his forces. This information can be used against future incursions, and potentially stop them altogether.
In the post-retcon runeterra nations seem very , very isolated, I'm not even sure any of the other nations know about the void or any other craziness happening in Shurima.
There are a few that could be anti-void though.
Kassadin does have followers, the "preservers of Valoran"... not sure if he still has them though :
Another point, Renekton was seen fighting rek'sai before so quite possible that 
are anti-void.
Institute agrees to unlock Xerath In return he destroys the void and lives peacefully( hopefully).
it might be because Runeterra has ALL KINDS of problems now. Bug monsters eat people trying to travel in Shurima, ghost gas from the Shadow Isles gets stronger and stronger every year as more and more people are stolen away, no one knows who won the Freljord yet, so an evil ice witch might have control of one of Runeterra's strongest factions, etc.
One guy screaming about a terrible incoming threat will just be told to get in line.
Kassadin needs to get himself a cult. Fight fire with fire, as they say.
Because, much like the Reapers (mass effect), even though there is 'evidence', national leaders cannot afford to deal with 'superstition' for the most part. They need hard evidence, backed by multiple sources and people, to constitute a response. It's the most prudent, and safe way, to command a nation.
Champion wise, why individual players are so unconcerned? It's hard to say. Maybe some are, but it's not part of their agenda. Quite a few mages are probably aware of the Void, just not directly affected by it yet.
