I can basically guess why they would omit Splatoon from this.
Splatoon ain't as popular here relatively than in Japan. And the public eye knows Mario and Duck hunt much better than they would Splatoon brands. If anything they'll have Splatoon brand shoes in Japan only.
Vans probably doesn't even give a shit about Splatoon. Mario is the brand strength.
They are very comfortable for me :/ I still have 3 pairs of slip-on shoes from Vans, no complaints. I mostly wear DC though, but I recently started with Puma and so far the 3 pairs I got a couple of months ago have been fine.