Privacy considerations were secondary and only came to light (from their perspective) during the increased scrutiny brought during COVID-19.
It was “cool” to use Slack until it became widely used, and then it was “cool” to use Keybase instead. Zoom is currently seen as “uncool” (E2EE screwup + widely used), so when they purchase “cool” Keybase, now Keybase automatically becomes “uncool” as well and people will look for something “cool” to migrate to next.
This isn’t a complete explanation of all possible reasons, but it’s absolutely a contributing factor.
EDIT: I predict Riot/Matrix will be the replacement “cool” for Keybase.
I've been juggling a lot of meetings between Zoom and my kid's school on different platforms, and the difference between Zoom and Google Meet is night and day. Schools are mostly switching to the latter because of the security concerns, but damn is it terrible. It's like Skype from 15 years ago.
Tech doesn't matter nearly so much as market. Marrying better tech chops to better market potential is a rather better investor storytime.
(Doesn't mean it'll work, doesn't mean Keybase tech will, or won't, survive. But the plave to be is Zoom's niche with Keybase's clue.)
Because Zoom is the buyer and they have the power. Sellers can make whatever promises they like (see: Whatsapp, Instagram) and it is reasonable to assume the buyer will have their way in the end.
Zoom will certainly use Keybase to improve their security overall. However, the rather obvious lack of commitment to existing users means there likely won't be any longterm.
My prediction: Zoom integrates well with keybase, there's a blog post that keybase is shutting down external services in a few months, the keybase founders leave and 1-2 years later, we hear of a new company they've founded.
In one way, good job Zoom for looking into security. In another way, I'm still looking at this awful UX that's buggy as hell and thinking it's gonna be a real slog for the keybase team to overcome that momentum.