I believe the reason it happened, in short, is that Take2 (the publisher) were really obsessed with the release being a surprise, at the cost of far too much.

For one, this meant that basically every job listing for the game never described what the game you’d even work on was. Most of the devs they got were juniors who:

1. were willing to sign more restrictive contracts without the confidence to push back
2. did not necessarily know much about the game, or even the genre (supposedly, besides Nate, only 1 dev was an active KSP1 player and another was aware of the game but never really played)
3. this game was their first sizeable project

For two, it meant that a lot of management roles were taken up by people from Take2 to enforce the secrecy (who also saw KSP as having franchise potential, but that’s a rant for another day). Few of them intimately understood what makes us dorky nerds enthusiastic about KSP.

This is also part of the reason they avoided talking to the KSP1 devs; they were afraid of some of them even hinting that a sequel was in the works. As to why they continued to not talk to them after announcing the game I’m not sure. Perhaps they were afraid they’d tell the uncomfortable truth that the game was making the same development mistakes as KSP1 and more.