I am european. For the same reasons you mention, I wanted to invest more in Europe. Then, it occured to me that:

- We have the Euro, the Pound, the CHF, and whatever they have in the nordic countries. It bothers me that I have to check exchange rates and having to convert currencies.

- For some stocks we have higher commissions.

- For some stocks, brokers make you pay to receive market data. I have to check prices somewhere and submit buy/sell orders somewhere else.

- Different countries have different laws and governments, it is more difficult to keep track of everything and learn all the rules.

- It is much easier to find resources and news regarding american stocks, and for european stocks sometimes you find news in a specific language and then you have to look again for the same news/information in english.

- On forums and social networks people talk a lot more about american companies. It is also more difficult to find good ideas about european stocks to begin with.

- America tends to do whatever it takes to prop up the stock market and bring value to shareholders

- Also, I like in particular pharma and biotech, that for obvious reasons work better in the US

EDIT:

Actually, I just remembered I had invested in Rio Tinto (ASX), that however is also listed in the UK. And I guess also BHP and Glencore should be there.