Of course there are other reasons. The United states is more affluent which means that more people will drive cars. The USA also has a lower population density which means that CO2 caused by transportation would be lower.

Many countries (such as Germany and some Eastern European countries) use natural gas in electricity generation in some places. This has lower carbon emissions, yet is substantially more expensive. It would also be interesting to consider CO2 imports and exports (if you buy a product that was manufactured in another country, then you import CO2). As an example, power hungry aluminium smelters are located where electricity is the cheapest. Yet everybody uses aluminium.

I would also like a citation for the “example, US's GDP is slightly less than the EU's but our CO2 emissions are double the EU.” statement.