Why during the 4G we chose to use the frequency band 2 - 8 GHz and not use 3 - 300 Ghz during deploying 4G network. Is there any recent development that enabled 5G which was unavailable while developing 4G?
The G stands for generation as stated in a reply above. Each generation represents a standard that it attempts to achieve, which is loosely agreed amongst a bunch of companies (e.g. Ericsson, Nokia and Samsung) in order to lay down a worldwide standard so that phone companies, telcos etc... can align their research and development.
'The goals of 5G technology can be summarized in the following value points:
1,000x increase in capacity
Support for 100+ billion connections
Up to 10Gbit/s speeds
Below 1ms latency'
Specifically the difference in 4G and 5G. 4G was mostly about increasing speeds and bandwidth in terms of increasing efficiency, using MIMO (multiple input and multiple output) strategies, duplexing, carrier aggregation ...
'5G is aiming on providing pervasive connectivity to lay grounds for fast and resilient access to the Internet users, whether they are on a top of a skyscraper or down under a subway station.' As you've alluded to there have been a number of advancements since 4G was released to the public. To implement 5G it will leverage the latest research in the area including beamforming, vRan and bunch more.
So as you can see mobile communications standards are more than just about frequency bands and includes improving technology for end to end communication. Hope that helps!
Reference: http://www.rfwireless-world.com/Terminology/4G-vs-5G-differe...