For convenience over security, it gave the choice of storing your private key on their servers. If Zoom was to one day shut Keybase down, your private key is now gone. Otherwise for security over convenience, if you store it on your own computer you're more likely to lose it.

Those trade-offs in Keybase's key management is probably why it did not take off including that they were on a VC payroll and not making any money at all from the service. It was only a matter of time that without Zoom, they would end up shutting down.