@Davide, the reason is that different countries would be using different custom fields for business identifier. It’s also possible to have multiple business identifiers and they would behave differently. There wouldn’t be generic one-fit-for-all custom field, that’s why I couldn’t convert it into a custom field.
For example, Australia has ABN which shows on certain reports. New Zealand has NZBN which will show on different reports.
Now, it’s possible for New Zealand company to have both ABN and NZBN and both fields are to be used in their own context. Generic
Business identifier custom field is not useful because I still need to know what it actually is.