Why is that so? Does anyone know whether there is a difference between Trento's German style beers and German German style beers?
To add to Leo's answer... Northern Italy is very much associated with Austro-Bavarian heritage. The province of South Tyrol is predominantly German speaking, and Trento is just south of that province. In addition, Weissbier is to Bavaria/Munich as Pretzels are to Philadelphia or Pizza is to New York.... Hmmm maybe that pretzels analogy was a bad idea since they are a German thing. Oh well, I think you get the point. You've got German people in Northern Italy making German beer. I would trust any Hefeweizen from Trentino over Blue Moon, that's for sure. Cheers!
I will resume here what I know about this topic.
One of the twenty regions of Italy, and one of the most northern, is Trentino-Alto Adige. It borders with Austria and, for some 20km, with Switzerland. Trentino-Alto Adige is divided in two provinces: province of Trento (Trentino, capital town Trento) and province of Bolzano/Bozen (Alto Adige/Südtirol, capital town Bolzano/Bozen). The latter is more northern than the former and is the proper bilingual area on that side (German and Italian are the two most spoken languages). Bolzano/Bozen is the most populated city council in its province and, curiously, the one with the highest rate of native Italian speakers. Unfortunately I can't reference this statistic that I heard in the years I was used to visit those places often.
Roughly speaking, the German or Austrian influence on culture and food and drinks is positively evident all around the region, becoming milder moving north to south.
Several Austrian and German beers are popular here: Stiegl, Spaten, Maisel's and many more. The interesting point about beers, but not unexpected: beverages from several foreigner brands and companies are distributed but also produced in Italy. While I was searching for the linked reference, that seems to agree, I was sure to remember that Weihenstephan, one of my favourite beers back to those mountains, was produced and bottled either in Germany or in Italy at the Forst brewery in Lagundo, in Alto Adige/Südtirol.
Is "because local people want to buy it" too glib? Where there's demand there is (or should be) supply. If the Italian versions of German style beers are anything like their British counterparts there will be a flavour difference (just as there is between the different beers that are produced within Germany) but the brewers will tend to (in their own unique fashion) try to be as true to the original style as possible. As this depends on the availability and quality of raw materials there will always be a difference, but that is true even within Germany.
In total: 1) because people like it. 2) not consistently but probably slightly more variation than in Germany.