Of course you are right that only a few members of the New Zealand main team are in the team, all the rest are new faces. However, this is everyone's expectation that New Zealand will play well.
The national boards have almost completely stopped international tours by 'A 'teams and the U-19 teams. As a result, apart from the main 4-5 batsmen, the others don't have sufficient experience of playing in hostile conditions. On top of that, the Bangladeshi spinners have been in terrific form recently. New Zealand board should have at least played a couple of warm-up matches before the outings against the national team. They could have at least got used to the conditions and avoided this humiliation.
@bryant.coleman the possible theory is they didn’t consider Bangladesh strong enough, and that’s why in their overconfidence they decided to not have any practice matches. Also this failure will serve them well, and it’ll help them understand where they need to be better and hopefully they’ll perform better in their upcoming matches.