Why can't I export a Contact Detail report to CSV?
The report you are trying to export (Contact Detail) does not currently support that feature. That particular report presents a variety of information that does not lend itself easily to be represented in a single spreadsheet.

You could use a similar report, such as Contact Summary, and export that.

Or you could use your programming expertise and creativity to find a way to make that report generate an exportable spreadsheet.

CiviCRM is an open-source project and new features come from community contributions of code or funding. The issue you referenced was closed due to lack of participation. Community members such as yourself could change this by stepping up to make it happen.