Because Skype/Mumble/TS/Vent were suggested for having a voice chat during LANs, Splatoon not having any ingame voice chat option right now.
The Skype mobile app is an alternative but it can have a really terrible audio quality depending on your device. Plus, some people might not even have a device that supports Skype, the probability is existent.
Seriously playing a team-based game at high level without voice chat is awful.
I also doubt Splatoon will get any LAN support, so people will have to rely on the venue's wifi. Ethernet adaptators are quite niche.
I really think we should stick to online wars for competition, unless Nintendo makes a move to change things in the right way.
Splatoon has a really good netcode, that's fine anyway.
EDIT :
@Agosta44
Of course I was only talking about LANs, since people seem to consider actual IRL tourneys for Splatoon.
I'm just stating how pointless it would be to run considering how poorly the game is conditionned for that.
Therefore, let's just consider online clan wars for this format debate.