Why are some AWS Regions disabled by default?
What is the main reason behind it?
It's because of History.
Prior to 2019, new regions simply 'appeared' on AWS Accounts as fully Enabled, so users could create resources in new regions immediately.
However, using more regions is not necessarily a good thing. Some companies prefer to have all their resources in one region or a few specific regions. It can be quite complex to write IAM permissions that limit user rights to specific regions, and it is made even more complex when new regions are added.
Therefore, in March 2019 AWS made a change such that 'new' regions will be disabled by default.
From Setting permissions to enable accounts for upcoming AWS Regions | AWS Security Blog:
AWS is changing the default behavior for all new Regions going forward. For new AWS Regions, you will opt-in to enable your account to operate in those Regions. This makes it easier for you to select the regions where you can create and manage AWS resources.
Additional permissions were defined to allow Admins to enable/disable specific regions.