Because of it’s short range and flaky receiving signal strength?
One would need two or more APs to cover the same area with acceptable 5GHz signal compared to one AP providing a 2.4GHz signal. Roaming between APs may lead to interruptions in playback (for Roon: Device vanishes and becomes available shortly after; playback stops/pauses until you press play again). Even if one doesn’t roam between APs, moving around is very bad for receiving signal strength (strongly dependent on actual orientation and heavily influenced by nearby obstacles like for example the user). Even for stationary use the receiving signal strength might be flaky while people roam around in the house, doors/windows get open and closed, other WiFi devices may also greatly influence the current WiFi distribution (hint: techniques like beamforming and the likes). The space in such small devices like Poly or 2go also seldom allow for good antenna placement and/or the use of multiple antennas.
So 2.4GHz WiFI is clearly the better option for mobile use (when one carries the Mojo/Poly or Hugo 2/2go around in the pouch while listening to music), the 2go got a Ethernet port for stationary use. It’s left up to the user how to make use of it. It doesn’t have to be used with a wired Ethernet connection directly to the router at the opposite end of the building. 
One can also use it in conjunction with other connection techniques as long as the adapter used provides a Ethernet port (EoP, FC, …). One can even connect it to a AP that works in client mode (a.k.a. Bridge Mode) and supports 5GHz WiFi, has a multitude of antennas (some models even have external ones that can be aligned for optimal results) and is designed and built as a WiFi device in the first place. Such a setup
- allows for a proper placement of the WiFi component independent of the placement of the audio device. It might be beneficial to not have a WiFi transmitter in direct vicinity to your audio or other electronic equipment.
- can be used to replace the often poor WiFi solutions in other devices (TV, Audio and AV equipment, Game Consoles, …) in the same Room too. Add a switch if needed.
PS: The forum is already full of people complaining about the bad results they got from integrated WiFi solutions (various Bluesound devices, KEF LS50w, Sonos gear, …). I appreciate it that Chord opted against that road leading nowhere.