Only needs one antenna to also do Bluetooth. Compatible with all routers except ones owned by techies that wish 2.4GHz would finally die.

My guess is it's because 5ghz WiFi is regulatory hell. I honestly can't think of a good way (off the top of my head) to make this easily usable for international users without making it

a) easy for them to interfere with primary 5ghz users Or

b) making multiple boards with an extra ROM chip ($$) telling people not to export them to other countries.

At least with 2.4ghz you can just stick to lower numbered channels.

Cost is the usual one. Presumably 5GHz requires another antenna, matching network, patent licenses, etc.