Why do you call some wines by region and other wines by grapes varietal? What is the reason behind?
This is a historical thing. In Europe in the ye olden days, there was no separation between where someone grew the grapes and what the wine was called, ie Burgundy, Chianti, Bordeaux, Rioja. In most of these regions, these grapes were selected over generations of growers to work perfectly in those regions.
Later on, maybe about 100 years ago, people started naming their wines similar names to these well known regions. Champagne is the classic example and probably the first to assert it's region name (aka appellation) around the world. Calling something Champagne in California did not make it Champagne. Champagne only comes from the Champagne region of France not Napa Valley in California.
It was intrinsically known what would grow where in Europe and mostly people didn't fiddle with it. Chardonnay and Pinot Noir grew well in Burgundy and that's pretty much all they grew. Then along came the New World. Mostly after WWII, growing regions all over the world started growing grapes and didn't have the history that many of these old European regions had and they wanted to tell people what was in the bottle so the rise of grape names came into being.
In Europe they created governing bodies to protect their appellations and making laws about how and what people could grow grapes. Protecting place names became a big deal. Now it's spread to many other things like tea, cheese, tomatoes (San Marzano), etc.
In the new world, they have the freedom to pretty much grow whatever they want, how they want and wherever they can so an emphasis on cute names and grape names is emphasis and places where grapes are grown are de-emphasized (OK, there are excepts like Napa Valley and Barossa Valley) As new world areas really figure out what is best grown in their regions you will see more emphasis on regional names and less on grapes.
It depends on the wine.
If you have a quality assurance label, and a strong relation between a particular region and a kind of wine, it's better to call wines by region.
The same is true if you can get different wines in different regions from the same grape varietal.
Also, if the wine is made from different grape varietals, calling by region is an obvious choice.
Just thing about Champagne and Franciacorta: they are well-known regions for sparkling wines, the wines are from mixed grapes, and the grapes are "almost" the same.