I'm not even american but still so proud of what humans can achieve.
I imagine its also because "SpaceX" is awkward to chant. Chanting too fast or with too many people and it will start sounding like "Space sex". Don't believe me? Say "Space X" ten times fast...
[1] http://www.esquire.com/news-politics/a16681/elon-musk-interv...
Source? He is a US citizen so I'm not sure what 'American' would mean otherwise.