Without going too far in to the politics of GNU/Linux, it's a bit messy. All the distros are different from each other, plus they're changing so rapidly that a book about one version is much less useful for managing another (think Ubuntu 14 -> 16, or Ubuntu 16 -> 18).

Also, while GNU/Linux is generally portable, many of the things you learn about Ubuntu will be different on what you'd run on a Raspberry Pi, or an UltraSPARC, or on a PowerPC.

NetBSD is the same OS on all the hardware it supports, so the system consistently does what it does regardless of where it's run. If you want to develop on a desktop and deploy on a Pi, for example, you'll have a much easier time with NetBSD :)