This is a great question. There are several reason why nations are losing their cultures so quickly.
First, the world is becoming a smaller and smaller place. With the development of technology, borders are being crossed all the time. For example, through transportation people are able to travel far more than ever before. Through trade, what is produced in one country is moved to another and with this, culture is being imported as well. In a word, globalization is making the world smaller.
Second, the reality of a common currency (Euro) for many European nations is also blurring boundaries and allowing for their to be more of a melding of cultures.
Finally, urbanization is also important. In other words, cities are resembling other cities more and more. There is a urban culture that is becoming more distinct. So, cities like New York, Tokyo, Berlin, Paris, London and others have a certain similarity, because they are cities. This, too, is weakening cultural identities.