Millimeter Wave is About Capacity

"The driving force behind 5G millimeter wave is fairly simple," says Joe Madden, Mobile Experts chief analyst . "It's the need for more capacity.” 

And there is only so much capacity to be wrung from low-band networks, for reasons of physics. signals at higher frequencies inherently can carry more data.

As a simple example, assume that every zero crossing of any wave represents the opportunity to code a bit (one or zero). Using a very simple coding, the total amount of bits that can be represented in one second of time hinges on the number of oscillations a wave makes over that period of time.

source: Wikimedia

Higher-frequency signals make more crossings, and therefore can represent more symbols. And that is why millimeter wave signals have such high bandwidth: there are simply so many more zero crossings in any unit of time, compared to lower-frequency signals (600 MHz to 800 MHz or 2-GHz, for example).

Also, channels vary in size, from 5 MHz up to perhaps 400 MHz. The wider the channel, the greater the capacity. That holds for Wi-Fi as well as mobile networks. This example using Wi-Fi illustrates the principle. Wider channels support more bandwidth because there are fewer guard bands between discrete channels. 

source: Araknis Networks

Attribute	Low Bands for 5G		Band 41	C-Band	mmWave
Frequency	600 MHz (T-Mobile)	700-1000 MHz (ATT/Verizon)	2.49-2.69GHz (Sprint)	3.7-4.2 GHz	24-39 GHz
Channel	20MHz	20MHz	60MHz	100MHz	400MHz
Typical Rate	40-80Mbps	40-80Mbps	100Mbps+	250-500Mbps	800Mbps+
Available	Now	Now	Now	2021~2022	Now
Coverage	Wide area	Wide area	Wide area	Wide area	Small area

source: Mobile Experts

At 600 MHz, 700 MHz and 800 MHz, mobile channels can range from 5 MHz to 20 MHz. In the mid-band region, channels might range from 60 MHz to 100 MHz. In the millimeter wave region, channels can reach 400 MHz. As you can see, capacity and frequency are directly related.