Three reasons:
-
The device does not perform well enough to handle the full gamut of Roon’s format support (this also applies to the original Pis), and uses an outdated instruction set.
-
There’s only one USB port, no built in networking, and no built-in audio output. This means networking will run over the Pi’s notoriously dodgy USB implementation, and people will be tempted to run audio over the same USB port through a hub–a configuration that we don’t feel is likely to be solid.
-
The full-size Pi 2/3 isn’t prohibitively expensive for individuals, especially when you consider the extra peripherals you’d need to buy to make a Pi-zero work. For embedding applications, there are far better options in a similar price range that don’t use dinosaur CPUs. For example, this $8 board has ethernet, USB, and a quad-core cortex-a7. By the time you got networking going on the Pi-Zero, you’d have outspent the difference.