Many devices dont need the speed provided by a 5ghz band connection and therefore dont have a 5ghz radio in them.
Example: A automatic door lock, a smoke alarm, etc. They arent streaming large multimedia data, therefore they can reduce their size and power requirement by only having one radio. It costs less to manufacture.
Typically you will find that if a device has a 5ghz band radio in it, it also has a 2.4 radio since it will retain backwards compatibility. You will rarely find a 5ghz band only device
2.4 gigs have longer wavelengths, and therefore can go farther in distance than 5ghz connections... but more devices use the older 2.4 band so that spectrum is more crowded.
Here is a good background article on this:
https://www.howtogeek.com/222249/whats-the-difference-between-2.4-ghz-and-5-ghz-wi-fi-and-which-shou...
