Bans aren't just about what's OP (though extreme exceptions like Kassa and Mord WILL be ban slammed.) In pro play people focus more on comps and banning what counters their comp outside of the occasional targeting a lane/player for bans. NV likely didn't ban Nidalee because they felt that Rek'Sai was more of a threat to their team. Svenskeren is also one of the better Rek'Sai players on LCS (my opinion only.)
NV isn't the best team and I don't watch them often unless they're against teams I enjoy watching (CLG, TL, FNC, OG) and my boyfriends teams (C9, TSM) so I don't know their playstyle very well. It's hard to tell what they were planning on outside of banning out the Rek'Sai that could flash + knockup the Kog and Ashe.
My opinion only: Rek'Sai is a very good ban for LCS because of her overall utility that can be abused. The tremor sense allows for tracking without direct vision while also being a tank and offering that utility. Nidalee has primarily damage and jungle control (less after her traps and hunt passive were nerfed.) Rek'Sai has not had her jungle control nerfed.
I know Taliyah, Shen, and Vlad also found themselves banned. Bjergsen is the only mid I've seen play Taliyah and it was to great success. Shen is very strong and does very well against ADCs that struggle with kiting (Kog'Maw.) Vlad is just... Vlad. It's not a wasted ban because of how many resources it takes to kill one is all I can get at.
NV probably should have banned Nidalee during Game 2.