Hold viols is all about clock skew and data path delay. After CTS, you got clock skew done(provided no clock routing gets changed afterwards), but routing isn't done yet. You can fix hold after CTS, but it could overkill your design. Imagine there is a path with hold viols after CTS, but because of the congestion, the routings on this path went scenic route and data path delay got slower than estimated wire delay from global route. Despite you didn't need to fix this path, since you didn't see this happening before routings,you might fix it unnecessarily.
Plus, you need to fix the viols caused by coupling cap after routing.