That's pretty much the only thing they aren't doing from the Splatoon launch, and with how character focused the game is I doubt that they wouldn't be popular. 

They probably couldn't figure out how to make the arms work or look right in amiibo form, or maybe they simply didn't want to bother doing it. Unless they're planning on releasing ARMS amiibo after the game comes out.

The arms would probably be used in ways they shouldn't be.

I think Nintendo has doubts that this game will be popular. I think they are taking a wait and see approach.

I imagine it may have to do with their ARMS.... xD

Perhaps they couldn't make them work as a toy without them being too fragile or something.

Were Splatoon's amiibo ready at launch? I thought they came a little bit afterwards. Though in that case they were probably still prepping them for eventual release. One reason was that amiibo was still new then and really hot so they were likely trying to include support for every Wii U game.

On the other hand, they might be prepping ARMS toys for release a little later and just haven't told us about it yet.

after they sold half a million...

Didn't stop them from making the Wii though. :p 

I want one of them Twintelle amiibo for... reasons.

It wouldn't be right if they did an amiibo for some and left out others, so I'd dont think they wanted to release 10 amiibo on 1 day. Although they could've done two (spring man and ribbon girl) without anyone complaining