Because carriers and manufacturers together are trying to find bands that work how we want during life conditions. For instance inside hospitals with multiple floors of concrete we need high frequency bands to keep the speed we want and need for today's uses especially with multiple devices are in the same vicinity. We need the high frequencies to get through barriers and connection quality in dense urban areas. But high frequencies don't travel far.

On top of health laws and regulations.

Lower frequencies travel further but the speeds aren't as fast and any barriers in between the device and tower or transmitter for low frequencies will interfere with objects. Which is why 2g is good for rural areas but it's insecure and antiquated for modern speed and usages like we expect.

*phone repair tech for the last few years.