In most cases this is because it is configured that way.
To change this, you need to edit the issue field configuration that the project uses.
1. Go to General - Settings - Issue configuration - Issue fields and look up the relevant issue field configuration. (If you only want to change for one project, you should copy that configuration, make the changes to the copy and then set the project to use the copied configuration instead.)
2. Find the row for the problematic issue field, and expand the section "Edit access" to the right.
3. In the column "Create issue", set the permissions to "Read/Write" for all user groups that should be able to edit this when creating an issue.
4. Save the issue field configuration.
Now, you should be able to set the issue field in question right away when creating a new issue.