The main reason is that grape juice naturally ferments into wine. If you put ‘fresh’ grape juice in a bucket for long enough, it will naturally convert into wine. It might not be good wine but, it will be wine nonetheless. Wine is typically 11-14% alcohol, while grape juice is naturally 20-26% sugar. This sugar range will naturally convert into the desired alcohol ranges, therefore in most instances, sugar does not need to be added to the grape juice. Almost all other fruit juices have a lower sugar than grape juice therefore sugar has to be added in order to make fruit wine.
Also grape juice is proper acidity to make wine. The total Acidity of Red grape juice is generally around 0.6 %, while white grape juice is 0.5%. This is the ideal range for a good clean fermentation of the wine. Other fruit juices have lower acidity therefore acidity needs to be added in order to complete the ferment.
Also that was likely a bit too scientific for the average reader. The important point is that grape juice is the idea juice to make wine with due to its natural characteristics.