First off, the reason is the city. Shenzhen is well-known for its economic development. This city has a lot of foreign companies, which means that foreigners are interested in investing money in Shenzhen. Second, Shenzhen University is among the top universities in China and it can be considered as the major University of the city. So if you want to learn Mandarin Chinese, this University is the best choice. Third, Shenzhen is located in southern China, and I've never been to southern China. Unfortunately (for foreigners who study Standard Chinese with simplified characters), there is a lot of Chinese dialects in China. Unlike dialects of Russian or English languages, Chinese dialects vary from Standard Mandarin so deeply, that they can be regarded as different languages. For example, Cantonese, which is common in southern China (especially in Hong Kong). But although Shenzhen is located south of Guangdong Province, near the Hong Kong, most people there speak Putonghua. Because there is a lot of chinese people who arrived from north and foreigners who study Putonghua. Actually I'm not against these dialects, I understand that it is part of Chinese culture. However, it is more convenient when not only qualified university teachers speak Putonghua but also other people who live in the city.