Nintendo doesn't attend the TGS and has already said that they won't be at this year's edition either, so the chances of NX being revealed there are zero.

That doesn't mean that Nintendo can't do an event on its own later this month or October.

Didn't Breath of the Wild always had a "2017" release date? 

Everything ever launches post Christmas

In some time after reveal "March 2017" was changed to "2017".

That would be odd given that the winner of the European Splatoon Tournament will win an NX, and that competition ends in March 2017. Do you think Nintendo will make them wait half a year to get their price? I find it unlikely.

Holiday 2017 would be a disaster, they need to get this thing out. I think there will be a Direct or event this month or next where they announce it

They never said that the NX would launch with BotW. It has only been an assumption.

Nintendo are patient, I think it could be holidays 2017

Yup.

What else could be the reason for it's delay? Wii U is dead in the water and the game looks finished so NX is the most likely cause.