One reason is the amount of grapes harvested per vine or per hectare of land.
Example 1: Here are three photos which show the difference between a quality-oriented and a quantity-oriented vineyard.
This is one of only two Tinta de Toro grapes on a vine in the Nuntia Vini vineyard. The average yield is 0.8 to 1 kg per vine. One bottle of wine produced from this vine costs more than € 25.00. Is this too expensive?
The result is a naturally pure wine, to which nothing except for sulfite has been added, but from which nothing has been removed. It has a high mineral content, a lot of resveratrol, and other phenols which are antioxidant and anticarcinogenic. With its full, concentrated taste, it provides true enjoyment and stores well.
Example 2: Here you can see a part of the yield of an Ugni blanc vine in the west of France. The owner of this vine will probably harvest more than 20 kg. The wine produced from this should cost no more than € 1. However a bottle will cost about € 5.00. A good bargain?
These white grapes look beautiful, but the result is a thin, insignificant, completely neutral wine with a very low mineral content. Presumably the sugar content of the grapes is so low that sugar has to be added to the must before fermentation. In the language of wine, this process is called "chaptalization", and it is done so that the alcohol content of the wine after fermentation will be high enough for it to actually be called a wine. Since so many grapes on the vine are unlikely to be completely ripe, they have a lot of acid, which has to be removed from the wine using the suitable compounds, or rectified concentrated grape must is added to it in order to neutralize the acidic taste.