Berlin is very popular in the US as a colourful, diverse city, a city of freedom with a special history. But the good price-performance ratio and the new aspect of it also attract American corporations and associations to the capital.
Moreover, Berlin is one of the only German cities that can keep up with the extent of American events: In November, for example, the OpenStack summit with 8,000 participants will be held in Berlin's CityCube. The choice and variety of locations for events of this size is important for US conventions.